Multi-channel echo cancel method, multi-channel sound transfer method, stereo echo canceller, stereo sound transfer apparatus and transfer function calculation apparatus

ABSTRACT

Stereo sound signals are reproduced directly from loudspeakers (SP(L), SP(R)). By using a sum signal and a difference signal of the stereo sound signals as a reference signal, and according to a cross spectrum calculation of the reference signal with a microphone-collected sound signal, calculation is performed to obtain transfer functions of four sound transfer systems between the loudspeakers (SP(L), SP(R)) and microphones (MC(L), MC(R)). The transfer functions obtained are subjected to inverse Fourier transform to obtain impulse responses, which are set in filter means ( 40 - 1  to  40 - 4 ) to create echo cancel signals and perform echo canceling. This solves the problem of an indefinite coefficient in the echo cancel technique of a multi-channel sound signal.

RELATED APPLICATIONS

[0001] This application is a continuation of PCT application No.PCT/JP02/06968, filed Jul. 10, 2002, which is based upon, and claimspriority from, Japanese Patent Application No. 2001-211279, filed Jul.11, 2001 and Japanese Patent Application No. 2002-179722, filed Jun. 20,2002.

TECHNICAL FIELD

[0002] This invention relates to an echo cancellation technique formulti-channel audio signals and a transfer function calculationtechnique, and solves a problem of indefinite coefficient by a newtechnique.

BACKGROUND ART

[0003] In two-way stereo audio transmission that is used inteleconferencing systems and so forth, the problem of indefinitecoefficient of echo cancellers has conventionally been pointed out and,for solving it, there have been proposed various techniques (see Journalof The Institute of Electronics, Information and CommunicationEngineers, vol. 81, No. 3, pp. 266-274, March 1998). As one of thetechniques for solving the problem of indefinite coefficient, there is amethod of reducing the interchannel correlation. As concrete techniquestherefor, there have conventionally been proposed the addition of randomnoise, the correlation removal by filters, the interchannel frequencyshift, the use of an interleave comb filter, the nonlinear processing(Laid-Open Patent Publication No. H10-190848) and so forth.

[0004] According to the foregoing conventional techniques, sinceoriginal stereo signals are subjected to processing and then reproduced,there has been a problem that deterioration more or less occurs inreproduced signals. Further, when the processing is complicated, a delayoccurs in the reproduced signals, so that there has been a problem ofdifficulty in conversation in the teleconference and so forth. Further,when the processing is complicated and the processing capability of aprocessing circuit is low, there have been those instances where it isdifficult to update a coefficient of an echo canceller in real timewhile carrying out the echo cancel processing.

[0005] This invention provides an echo cancellation technique formulti-channel audio signals and a transfer function calculationtechnique that have solved the foregoing problems in the conventionaltechniques.

DISCLOSURE OF THE INVENTION

[0006] [Inventions of Claims 1 to 8 and Inventions Relating to SuchInventions]

[0007] A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsinputted from an outside and reproduced by said respective loudspeakersand having a correlation with each other are collected by saidmicrophones, individual transfer functions of said plurality of audiotransfer systems or a plurality of composite transfer functions obtainedby suitably combining said individual transfer functions are estimatedso as to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to corresponding individual signals to be reproduced bysaid respective loudspeakers or a plurality of composite signalsobtained by suitably combining said individual signals, and said echocancel signals are subtracted from corresponding individual collectedaudio signals of said one or plurality of microphones, or a plurality ofcomposite signals obtained by suitably combining said individualcollected audio signals, thereby performing echo cancellation, andwherein, using as reference signals (representing those signals that arereferred to for estimating the transfer functions or the compositetransfer functions) a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combining saidmulti-channel audio signals and which have a lower correlation with eachother than that between said multi-channel audio signals (e.g. suitablycombining said multi-channel audio signals to produce a plurality oflow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals and using a setof said plurality of low-correlation composite signals as referencesignals, or directly inputting a set of a plurality of low-correlationcomposite signals which correspond to signals obtained by suitablycombining said multi-channel audio signals and which have a lowercorrelation with each other than that between said multi-channel audiosignals and using the set of said plurality of low-correlation compositesignals as reference signals, or the like), individual transferfunctions of the respective audio transfer systems or a plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions are respectively derived, thereby to setcorresponding filter characteristics. According to this invention, usingas reference signals a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combiningmulti-channel audio signals having a correlation therebetween and whichhave a lower correlation with each other than that between suchmulti-channel audio signals, individual transfer functions of therespective audio transfer systems or a plurality of composite transferfunctions obtained by suitably combining such individual transferfunctions are respectively derived, and corresponding filtercharacteristics are set, thereby to enable echo cancellation. Inaccordance therewith, since the multi-channel audio signals can bereproduced from the loudspeakers with no or less processing, whichinduces deterioration, applied to the multi-channel audio signals,excellent reproduced tone quality can be achieved. Further, there is noor only a small delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. The calculation of respectively deriving the individualtransfer functions of the respective audio transfer systems or theplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions, using as the reference signals theset of the plurality of low-correlation composite signals, may be, forexample, a calculation of respectively deriving the individual transferfunctions of the respective audio transfer systems or the plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions, based on a cross-spectrum calculationbetween the plurality of low-correlation composite signals and theindividual collected audio signals of the microphones, or the pluralityof composite signals obtained by suitably combining said individualcollected audio signals. Further, the calculation of respectivelyderiving the individual transfer functions of said plurality of audiotransfer systems or the plurality of composite transfer functionsobtained by suitably combining said individual transfer functions, basedon said cross-spectrum calculation, may be, for example, a calculationof respectively deriving the individual transfer functions of saidplurality of audio transfer systems or the plurality of compositetransfer functions obtained by suitably combining said individualtransfer functions, by combining said multi-channel audio signalsthrough addition or subtraction to produce a plurality oflow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals, deriving crossspectra between said plurality of low-correlation composite signals andthe individual collected audio signals of the microphones, or theplurality of composite signals obtained by suitably combining saidindividual collected audio signals, and ensemble-averaging them in apredetermined time period per cross spectrum.

[0008] A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsinputted from an outside and reproduced by said respective loudspeakersand having a correlation with each other are collected by saidmicrophones, individual transfer functions of said plurality of audiotransfer systems or a plurality of composite transfer functions obtainedby suitably combining said individual transfer functions are estimatedso as to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to corresponding individual signals to be reproduced bysaid respective loudspeakers or a plurality of composite signalsobtained by suitably combining said individual signals, and said echocancel signals are subtracted from corresponding individual collectedaudio signals of said one or plurality of microphones, or a plurality ofcomposite signals obtained by suitably combining said individualcollected audio signals, thereby performing echo cancellation, wherein,using as reference signals a set of a plurality of low-correlationcomposite signals which correspond to signals obtained by suitablycombining said multi-channel audio signals and which have a lowercorrelation with each other than that between said multi-channel audiosignals (e.g. suitably combining said multi-channel audio signals toproduce a plurality of low-correlation composite signals having a lowercorrelation with each other than that between said multi-channel audiosignals and using a set of said plurality of low-correlation compositesignals as reference signals, or directly inputting a set of a pluralityof low-correlation composite signals which correspond to signalsobtained by suitably combining said multi-channel audio signals andwhich have a lower correlation with each other than that between saidmulti-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),estimated errors of individual transfer functions of the respectiveaudio transfer systems or a plurality of composite transfer functionsobtained by suitably combining said individual transfer functions arerespectively derived, thereby to update corresponding filtercharacteristics to values that cancel said estimated errors. Accordingto this invention, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining multi-channel audio signals having a correlationtherebetween and which have a lower correlation with each other thanthat between such multi-channel audio signals, estimated errors ofindividual transfer functions of the respective audio transfer systemsor a plurality of composite transfer functions obtained by suitablycombining such individual transfer functions are respectively derived soas to successively update the corresponding filter characteristics tovalues that cancel such estimated errors, thereby to enable echocancellation. In accordance therewith, since the multi-channel audiosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the multi-channelaudio signals, excellent reproduced tone quality can be achieved.Further, there is no or only a small delay in reproduced signals. Thus,when applying to the teleconferencing system or the like, naturalconversation can be conducted. Further, it is possible to update thefilter characteristics in real time. The calculation of respectivelyderiving the estimated errors of the individual transfer functions ofthe respective audio transfer systems or the plurality of compositetransfer functions obtained by suitably combining said individualtransfer functions, using as the reference signals the set of theplurality of low-correlation composite signals, may be, for example, acalculation of respectively deriving the estimated errors of theindividual transfer functions of the respective audio transfer systemsor the plurality of composite transfer functions obtained by suitablycombining said individual transfer functions, based on a cross-spectrumcalculation between said plurality of low-correlation composite signalsand echo cancel error signals obtained by subtracting the echo cancelsignals from the corresponding individual collected audio signals ofsaid one or plurality of microphones, or the plurality of compositesignals obtained by suitably combining said individual collected audiosignals. Further, the calculation of respectively deriving the estimatederrors of the individual transfer functions of said plurality of audiotransfer systems or the plurality of composite transfer functionsobtained by suitably combining said individual transfer functions, basedon said cross-spectrum calculation, may be, for example, a calculationof respectively deriving the estimated errors of the individual transferfunctions of said plurality of audio transfer systems or the pluralityof composite transfer functions obtained by suitably combining saidindividual transfer functions, by combining said multi-channel audiosignals through addition or subtraction to produce a plurality oflow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals, deriving crossspectra between said plurality of low-correlation composite signals andthe echo cancel error signals obtained by subtracting the echo cancelsignals from the corresponding individual collected audio signals ofsaid one or plurality of microphones, or the plurality of compositesignals obtained by suitably combining said individual collected audiosignals, and ensemble-averaging them in a predetermined time period percross spectrum. Further, the correlation between said plurality oflow-correlation composite signals is detected and, when a value of saidcorrelation is no less than a prescribed value, updating of said filtercharacteristics is stopped, thereby to prevent the echo cancel errorsignals from unexpectedly increasing.

[0009] A multi-channel sound transfer method of this invention is suchthat, with respect to two spaces each forming said plurality of audiotransfer systems, any of the foregoing multi-channel echo cancel methodsis carried out, so that the multi-channel audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the multi-channel audiotransmission with reduced echo cancellation can be performed between twospots, which, for example, can be applied to the teleconferencing systemor the like.

[0010] A stereo echo cancel method of this invention is a methodwherein, with respect to a space provided therein with two loudspeakersand one or two microphones and forming two or four audio transfersystems in which stereo sounds reproduced by said respectiveloudspeakers are collected by said microphones, individual transferfunctions of said two or four audio transfer systems or a plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions are estimated so as to set correspondingfilter characteristics, respectively, echo cancel signals arerespectively produced by giving said set filter characteristics tocorresponding individual signals to be reproduced by said respectiveloudspeakers or a plurality of composite signals obtained by suitablycombining said individual signals, and said echo cancel signals aresubtracted from corresponding individual collected audio signals of saidone or two microphones, or a plurality of composite signals obtained bysuitably combining said individual collected audio signals, therebyperforming echo cancellation, and wherein, using a sum signal and adifference signal of said stereo audio signals as reference signals,individual transfer functions of said two or four audio transfer systemsor a plurality of composite transfer functions obtained by suitablycombining said individual transfer functions are respectively derived,thereby to set corresponding filter characteristics. According to thisinvention, since the sum signal and the difference signal of the stereoaudio signals have a low correlation therebetween, the transferfunctions of the two or four audio transfer systems or their compositetransfer functions are respectively derived using the sum signal and thedifference signal as reference signals, so as to set the correspondingfilter characteristics, thereby to enable echo cancellation. Inaccordance therewith, since the stereo signals can be reproduced fromthe loudspeakers with no or less processing, which inducesdeterioration, applied to the stereo signals, excellent reproduced tonequality can be achieved. Further, there is no or only a small delay inreproduced signals. Thus, when applying to the teleconferencing systemor the like, natural conversation can be conducted. The calculation ofrespectively deriving the individual transfer functions of said two orfour audio transfer systems or the plurality of composite transferfunctions obtained by suitably combining said individual transferfunctions, using the sum signal and the difference signal of said stereoaudio signals as the reference signals, may be, for example, acalculation of respectively deriving the individual transfer functionsof said two or four audio transfer systems or the plurality of compositetransfer functions obtained by suitably combining said individualtransfer functions, based on a cross-spectrum calculation between thesum signal and the difference signal, and the individual collected audiosignals of the microphones, or the plurality of composite signalsobtained by suitably combining said individual collected audio signals.Further, the calculation of respectively deriving the individualtransfer functions of said two or four audio transfer systems or theplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions, based on said cross-spectrumcalculation, may be, for example, a calculation of respectively derivingthe individual transfer functions of said two or four audio transfersystems or the plurality of composite transfer functions obtained bysuitably combining said individual transfer functions, by deriving crossspectra between the sum signal and the difference signal of said stereoaudio signals and the individual collected audio signals of themicrophones or the plurality of composite signals obtained by suitablycombining said individual collected audio signals, andensemble-averaging them in a predetermined time period per crossspectrum.

[0011] A stereo echo cancel method of this invention is a methodwherein, with respect to a space provided therein with two loudspeakersand one or two microphones and forming two or four audio transfersystems in which stereo sounds reproduced by said respectiveloudspeakers are collected by said microphones, individual transferfunctions of said two or four audio transfer systems or a plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions are estimated so as to set correspondingfilter characteristics, respectively, echo cancel signals arerespectively produced by giving said set filter characteristics tocorresponding individual signals to be reproduced by said respectiveloudspeakers or a plurality of composite signals obtained by suitablycombining said individual signals, and said echo cancel signals aresubtracted from corresponding individual collected audio signals of saidone or two microphones, or a plurality of composite signals obtained bysuitably combining said individual collected audio signals, therebyperforming echo cancellation, and wherein, using a sum signal and adifference signal of said stereo audio signals as reference signals,estimated errors of individual transfer functions of said two or fouraudio transfer systems or a plurality of composite transfer functionsobtained by suitably combining said individual transfer functions arerespectively derived, thereby to update corresponding filtercharacteristics to values that cancel said estimated errors. Accordingto this invention, since the sum signal and the difference signal of thestereo audio signals have a low correlation therebetween, the estimatederrors of the transfer functions of the two or four audio transfersystems or their composite transfer functions are respectively derivedusing the sum signal and the difference signal as reference signals, soas to successively update the corresponding filter characteristics tothe values that cancel the estimated errors, thereby to enable echocancellation. In accordance therewith, since the stereo signals can bereproduced from the loudspeakers with no or less processing, whichinduces deterioration, applied to the stereo signals, excellentreproduced tone quality can be achieved. Further, there is no or only asmall delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. The calculation of respectively deriving the estimated errorsof the individual transfer functions of said two or four audio transfersystems or the plurality of composite transfer functions obtained bysuitably combining said individual transfer functions, using the sumsignal and the difference signal of said stereo audio signals as thereference signals, may be, for example, a calculation of respectivelyderiving the estimated errors of the individual transfer functions ofsaid two or four audio transfer systems or the plurality of compositetransfer functions obtained by suitably combining said individualtransfer functions, based on a cross-spectrum calculation between thesum signal and the difference signal of said stereo audio signals andrespective echo cancel error signals obtained by subtracting thecorresponding echo cancel signals from the individual collected audiosignals of said one or two microphones, or the plurality of compositesignals obtained by suitably combining said individual collected audiosignals. The calculation of respectively deriving the estimated errorsof the individual transfer functions of said two or four audio transfersystems or the plurality of composite transfer functions obtained bysuitably combining said individual transfer functions, based on thecross-spectrum calculation between the sum signal and the differencesignal of said stereo audio signals and said echo cancel error signals,may be, for example, a calculation of respectively deriving theestimated errors of the individual transfer functions of said two orfour audio transfer systems or the plurality of composite transferfunctions obtained by suitably combining said individual transferfunctions, by deriving cross spectra between the sum signal and thedifference signal of said stereo audio signals and said echo cancelerror signals, and ensemble-averaging them in a predetermined timeperiod per cross spectrum. Further, the correlation between the sumsignal and the difference signal of said stereo audio signals isdetected and, when a value of said correlation is no less than aprescribed value, updating of said filter characteristics is stopped,thereby to prevent the echo cancel error signals from unexpectedlyincreasing.

[0012] A stereo audio transmission method of this invention is suchthat, with respect to two spaces each forming said four audio transfersystems, any of the foregoing multi-channel echo cancel methods iscarried out, so that the stereo audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the stereo audio transmission withreduced echo cancellation can be performed between two spots, which, forexample, can be applied to the teleconferencing system or the like.

[0013] In this invention, the plurality of composite signals obtained bysuitably combining the individual signals to be reproduced by therespective loudspeakers, and the plurality of low-correlation compositesignals used as reference signals {in this specification,“low-correlation composite signals” is used as including the meaning ofuncorrelated signals (uncorrelated composite signals)} may be, forexample, common signals.

[0014] [Inventions of Claims 9 to 21 and Inventions Relating to SuchInventions]

[0015] A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel sounds (e.g.multi-channel stereo sounds such as two-channel, four-channel, . . . )inputted from an outside and reproduced by said respective loudspeakersand having a correlation with each other are collected by saidmicrophones, transfer functions of said plurality of audio transfersystems are estimated so as to set corresponding filter characteristics,respectively, echo cancel signals are respectively produced by givingsaid filter characteristics to corresponding signals to be reproduced bysaid respective loudspeakers, and said echo cancel signals aresubtracted from corresponding collected audio signals of said one orplurality of microphones, thereby performing echo cancellation, andwherein, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals (e.g. suitably combining said multi-channel audio signalsto produce a plurality of low-correlation composite signals having alower correlation with each other than that between said multi-channelaudio signals and using a set of said plurality of low-correlationcomposite signals as reference signals, or directly inputting a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),transfer functions of the respective audio transfer systems arerespectively derived, thereby to set corresponding filtercharacteristics. According to this invention, using as reference signalsa set of a plurality of low-correlation composite signals whichcorrespond to signals obtained by suitably combining multi-channel audiosignals having a correlation therebetween and which have a lowercorrelation with each other than that between such multi-channel audiosignals, the transfer functions of the respective audio transfer systemsare respectively derived, and corresponding filter characteristics areset, thereby to enable echo cancellation. In accordance therewith, sincethe multi-channel audio signals can be reproduced from the loudspeakerswith no or less processing, which induces deterioration, applied to themulti-channel audio signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. The calculation of respectivelyderiving the transfer functions of the respective audio transfer systemsusing as the reference signals the set of the plurality oflow-correlation composite signals, may be, for example, a calculation ofrespectively deriving the transfer functions of the respective audiotransfer systems based on a cross-spectrum calculation between theplurality of low-correlation composite signals and the respectivemicrophone collected audio signals. Further, the calculation ofrespectively deriving the transfer functions of said plurality of audiotransfer systems based on said cross-spectrum calculation, may be, forexample, a calculation of combining said multi-channel audio signalsthrough addition or subtraction to produce a plurality oflow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals, deriving crossspectra between said plurality of low-correlation composite signals andthe respective microphone collected audio signals, andensemble-averaging them in a predetermined time period per crossspectrum to derive a plurality of kinds of composite transfer functionsobtained by combining transfer functions of a plurality of suitablesystems among said plurality of audio transfer systems, thereby toderive transfer functions of said plurality of audio transfer systemsbased on said plurality of kinds of composite transfer functions.

[0016] A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsinputted from an outside and reproduced by said respective loudspeakersand having a correlation with each other are collected by saidmicrophones, transfer functions of said plurality of audio transfersystems are estimated so as to set corresponding filter characteristics,respectively, echo cancel signals are respectively produced by givingsaid filter characteristics to corresponding signals to be reproduced bysaid respective loudspeakers, and said echo cancel signals aresubtracted from corresponding collected audio signals of said one orplurality of microphones, thereby performing echo cancellation, andwherein, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals (e.g. suitably combining said multi-channel audio signalsto produce a plurality of low-correlation composite signals having alower correlation with each other than that between said multi-channelaudio signals and using a set of said plurality of low-correlationcomposite signals as reference signals, or directly inputting a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),estimated errors of transfer functions of the respective audio transfersystems are respectively derived, thereby to update corresponding filtercharacteristics to values that cancel said estimated errors. Accordingto this invention, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining multi-channel audio signals having a correlationtherebetween and which have a lower correlation with each other thanthat between such multi-channel audio signals, the estimated errors ofthe transfer functions of the respective audio transfer systems arerespectively derived so as to successively update the correspondingfilter characteristics to the values that cancel said estimated errors,thereby to enable echo cancellation. In accordance therewith, since themulti-channel audio signals can be reproduced from the loudspeakers withno or less processing, which induces deterioration, applied to themulti-channel audio signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. It is also possible to update thefilter characteristics in real time. The filter characteristics can beupdated, for example, per suitably determined prescribed time period(e.g. time period of performing said ensemble averaging). Thecalculation of respectively deriving the estimated errors of thetransfer functions of the respective audio transfer systems using theset of the plurality of low-correlation composite signals as thereference signals, may be, for example, a calculation of respectivelyderiving the estimated errors of the transfer functions of therespective audio transfer systems based on a cross-spectrum calculationbetween said plurality of low-correlation composite signals and echocancel error signals obtained by subtracting the corresponding echocancel signals from the collected audio signals of said one or pluralityof microphones. Further, the calculation of respectively deriving theestimated errors of the transfer functions of said plurality of audiotransfer systems based on said cross-spectrum calculation, may be, forexample, a calculation of combining said multi-channel audio signalsthrough addition or subtraction to produce a plurality oflow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals, deriving crossspectra between said plurality of low-correlation composite signals andthe echo cancel error signals obtained by subtracting the correspondingecho cancel signals from the collected audio signals of said one orplurality of microphones, and ensemble-averaging them in a predeterminedtime period per cross spectrum to derive a plurality of kinds oftransfer function composite estimated errors obtained by combiningestimated errors of transfer functions of a plurality of suitablesystems among said plurality of audio transfer systems, thereby toderive estimated errors of the transfer functions of said plurality ofaudio transfer systems based on said plurality of kinds of transferfunction composite estimated errors. Further, the correlation betweensaid plurality of low-correlation composite signals is detected and,when a value of said correlation is no less than a prescribed value,updating of said filter characteristics is stopped, thereby to preventthe echo cancel error signals from unexpectedly increasing.

[0017] Further, the calculation of respectively deriving the transferfunctions of said plurality of audio transfer systems based on saidcross-spectrum calculation, may be, for example, a calculation ofproducing a plurality of mutually orthogonal uncorrelated compositesignals by applying a principal component analysis to said multi-channelaudio signals, deriving cross spectra between said plurality ofuncorrelated composite signals and the respective microphone collectedaudio signals, and ensemble-averaging them in a predetermined timeperiod per cross spectrum, thereby to derive the transfer functions ofsaid plurality of audio transfer systems based on the ensemble-averagedvalues.

[0018] Further, the calculation of respectively deriving the estimatederrors of the transfer functions of said plurality of audio transfersystems based on said cross-spectrum calculation, may be, for example, acalculation of producing a plurality of mutually orthogonal uncorrelatedcomposite signals by applying a principal component analysis to saidmulti-channel audio signals, deriving cross spectra between saidplurality of uncorrelated composite signals and the echo cancel errorsignals obtained by subtracting the corresponding echo cancel signalsfrom the collected audio signals of said one or plurality ofmicrophones, and ensemble-averaging them in a predetermined time periodper cross spectrum, thereby to derive the estimated errors of thetransfer functions of said plurality of audio transfer systems based onthe ensemble-averaged values. In this case, it may be arranged thatdouble talk in which sounds other than those reproduced by saidloudspeakers are inputted into said microphones is detected and, whenthe double talk is detected, an update period of said filtercharacteristics is made relatively longer, whereas, when the double talkis not detected, the update period of said filter characteristics ismade relatively shorter, so that it is possible to fully converge theestimated errors when the double talk exists, and further, quicken theconvergence of the estimated errors when there is no double talk.

[0019] A multi-channel sound transfer method of this invention is suchthat, with respect to two spaces each forming said plurality of audiotransfer systems, any of the foregoing multi-channel echo cancel methodsis carried out, so that the multi-channel audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the multi-channel audiotransmission with reduced echo cancellation can be performed between twospots, which, for example, can be applied to the teleconferencing systemor the like.

[0020] A stereo echo cancel method of this invention is a methodwherein, with respect to a space provided therein with two loudspeakersand one or two microphones and forming two or four audio transfersystems in which stereo sounds reproduced by said respectiveloudspeakers are collected by said microphones, transfer functions ofsaid two or four audio transfer systems are estimated so as to setcorresponding filter characteristics, respectively, echo cancel signalsare respectively produced by giving said filter characteristics tocorresponding signals to be reproduced by said respective loudspeakers,and said echo cancel signals are subtracted from corresponding collectedaudio signals of said one or two microphones, thereby performing echocancellation, and wherein, using a sum signal and a difference signal ofsaid stereo audio signals as reference signals, transfer functions ofsaid two or four audio transfer systems are respectively derived,thereby to set corresponding filter characteristics. According to thisinvention, since the sum signal and the difference signal of the stereoaudio signals have a low correlation therebetween, the transferfunctions of the two or four audio transfer systems are respectivelyderived using the sum signal and the difference signal as referencesignals, so as to set the corresponding filter characteristics, therebyto enable echo cancellation. In accordance therewith, since the stereosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the stereo signals,excellent reproduced tone quality can be achieved. Further, there is noor only a small delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. The calculation of respectively deriving the transferfunctions of said two or four audio transfer systems using the sumsignal and the difference signal of said stereo audio signals as thereference signals, may be, for example, a calculation of respectivelyderiving the transfer functions of said two or four audio transfersystems based on a cross-spectrum calculation between the sum signal andthe difference signal, and the respective microphone collected audiosignals. Further, the calculation of respectively deriving the transferfunctions of said two or four audio transfer systems based on saidcross-spectrum calculation, may be, for example, a calculation ofderiving cross spectra between the sum signal and the difference signalof said stereo audio signals and the respective microphone collectedaudio signals, and ensemble-averaging them in a predetermined timeperiod per cross spectrum to derive a plurality of kinds of compositetransfer functions obtained by combining transfer functions of aplurality of suitable systems among said two or four audio transfersystems, thereby to derive transfer functions of said two or four audiotransfer systems based on said plurality of kinds of composite transferfunctions. Further, the cross-spectrum calculation in case of the foursystems may calculate, for example, respective cross spectra betweensaid sum signal and the first microphone collected audio signal, betweensaid sum signal and the second microphone collected audio signal,between said difference signal and the first microphone collected audiosignal, and between said difference signal and the second microphonecollected audio signal. Further, said composite transfer functions mayinclude, for example, the first composite transfer function that is thesum of a transfer function between the first loudspeaker and the firstmicrophone and a transfer function between the second loudspeaker andthe first microphone, the second composite transfer function that is adifference between the transfer function between the first loudspeakerand the first microphone and the transfer function between the secondloudspeaker and the first microphone, the third composite transferfunction that is the sum of a transfer function between the firstloudspeaker and the second microphone and a transfer function betweenthe second loudspeaker and the second microphone, and the fourthcomposite transfer function that is a difference between the transferfunction between the first loudspeaker and the second microphone and thetransfer function between the second loudspeaker and the secondmicrophone. The calculation of deriving the transfer functions of saidfour audio transfer systems may include, for example, a calculation ofderiving a transfer function of the first audio transfer system from thesum of the first composite transfer function and the second compositetransfer function, a calculation of deriving a transfer function of thesecond audio transfer system from a difference between the firstcomposite transfer function and the second composite transfer function,a calculation of deriving a transfer function of the third audiotransfer system from the sum of the third composite transfer functionand the fourth composite transfer function, and a calculation ofderiving a transfer function of the fourth audio transfer system from adifference between the third composite transfer function and the fourthcomposite transfer function.

[0021] A stereo echo cancel method of this invention is a methodwherein, with respect to a space provided therein with two loudspeakersand one or two microphones and forming two or four audio transfersystems in which stereo sounds reproduced by said respectiveloudspeakers are collected by said microphones, transfer functions ofsaid two or four audio transfer systems are estimated so as to setcorresponding filter characteristics, respectively, echo cancel signalsare respectively produced by giving said filter characteristics tocorresponding signals to be reproduced by said respective loudspeakers,and said echo cancel signals are subtracted from corresponding collectedaudio signals of said one or two microphones, thereby performing echocancellation, and wherein, using a sum signal and a difference signal ofsaid stereo audio signals as reference signals, estimated errors oftransfer functions of said two or four audio transfer systems arerespectively derived, thereby to update corresponding filtercharacteristics to values that cancel said estimated errors. Accordingto this invention, since the sum signal and the difference signal of thestereo audio signals have a low correlation therebetween, the estimatederrors of the transfer functions of the two or four audio transfersystems are respectively derived using the sum signal and the differencesignal as reference signals, so as to successively update thecorresponding filter characteristics to the values that cancel theestimated errors, thereby to enable echo cancellation. In accordancetherewith, since the stereo signals can be reproduced from theloudspeakers with no or less processing, which induces deterioration,applied to the stereo signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. It is also possible to update theecho cancel coefficients (filter characteristics) in real time. Thefilter characteristics can be updated, for example, per suitablydetermined prescribed time period (e.g. time period of performing saidensemble averaging). The calculation of respectively deriving theestimated errors of the transfer functions of said two or four audiotransfer systems using the sum signal and the difference signal of saidstereo audio signals as the reference signals, may be, for example, acalculation of respectively deriving the estimated errors of thetransfer functions of said two or four audio transfer systems based on across-spectrum calculation between the sum signal and the differencesignal of said stereo audio signals and respective echo cancel errorsignals obtained by subtracting the corresponding echo cancel signalsfrom the collected audio signals of said one or two microphones.Further, the calculation of respectively deriving the estimated errorsof the transfer functions of said two or four audio transfer systemsbased on the cross-spectrum calculation between the sum signal and thedifference signal of said stereo audio signals and said echo cancelerror signals, may be, for example, a calculation of deriving crossspectra between the sum signal and the difference signal of said stereoaudio signals and said echo cancel error signals, and ensemble-averagingthem in a predetermined time period per cross spectrum to derive aplurality of kinds of transfer function composite estimated errorsobtained by combining estimated errors of transfer functions of aplurality of suitable systems among said two or four audio transfersystems, thereby to derive estimated errors of the transfer functions ofsaid two or four audio transfer systems based on said plurality of kindsof transfer function composite estimated errors. Further, thecross-spectrum calculation in case of the four systems may calculate,for example, respective cross spectra between said sum signal and thefirst echo cancel error signal, between said sum signal and the secondecho cancel error signal, between said difference signal and the firstecho cancel error signal, and between said difference signal and thesecond echo cancel error signal. Further, said transfer functioncomposite estimated errors may include, for example, the first transferfunction composite estimated error that is the sum of an estimated errorof a transfer function between the first loudspeaker and the firstmicrophone and an estimated error of a transfer function between thesecond loudspeaker and the first microphone, the second transferfunction composite estimated error that is a difference between theestimated error of the transfer function between the first loudspeakerand the first microphone and the estimated error of the transferfunction between the second loudspeaker and the first microphone, thethird transfer function composite estimated error that is the sum of anestimated error of a transfer function between the first loudspeaker andthe second microphone and an estimated error of a transfer functionbetween the second loudspeaker and the second microphone, and the fourthtransfer function composite estimated error that is a difference betweenthe estimated error of the transfer function between the firstloudspeaker and the second microphone and the estimated error of thetransfer function between the second loudspeaker and the secondmicrophone. The calculation of deriving the estimated errors of thetransfer functions of said four audio transfer systems may include, forexample, a calculation of deriving an estimated error of a transferfunction of the first audio transfer system from the sum of the firsttransfer function composite estimated error and the second transferfunction composite estimated error, a calculation of deriving anestimated error of a transfer function of the second audio transfersystem from a difference between the first transfer function compositeestimated error and the second transfer function composite estimatederror, a calculation of deriving an estimated error of a transferfunction of the third audio transfer system from the sum of the thirdtransfer function composite estimated error and the fourth transferfunction composite estimated error, and a calculation of deriving anestimated error of a transfer function of the fourth audio transfersystem from a difference between the third transfer function compositeestimated error and the fourth transfer function composite estimatederror. Further, the correlation between the sum signal and thedifference signal of said stereo audio signals is detected and, when avalue of said correlation is no less than a prescribed value, updatingof said filter characteristics is stopped, thereby to prevent the echocancel error signals from unexpectedly increasing.

[0022] A stereo echo cancel method of this invention is a methodwherein, with respect to a space provided therein with two loudspeakersand one or two microphones and forming two or four audio transfersystems in which stereo sounds reproduced by said respectiveloudspeakers are collected by said microphones, transfer functions ofsaid two or four audio transfer systems are estimated so as to setcorresponding filter characteristics, respectively, echo cancel signalsare respectively produced by giving said filter characteristics tocorresponding signals to be reproduced by said respective loudspeakers,and said echo cancel signals are subtracted from corresponding collectedaudio signals of said one or two microphones, thereby performing echocancellation, and wherein a principal component analysis is applied tosaid stereo audio signals to produce two uncorrelated composite signalsthat are orthogonal to each other, and transfer functions of said two orfour audio transfer systems are respectively derived using a set of saidtwo uncorrelated composite signals as reference signals, thereby to setcorresponding filter characteristics. According to this invention, sincethe mutually orthogonal two signals produced by applying the principalcomponent analysis to the stereo audio signals are uncorrelated witheach other, the transfer functions of said two or four audio transfersystems are respectively derived using such two signals, thereby to setthe corresponding filter characteristics to enable echo cancellation. Inaccordance therewith, since the stereo signals can be reproduced fromthe loudspeakers with no or less processing, which inducesdeterioration, applied to the stereo signals, excellent reproduced tonequality can be achieved. Further, there is no or only a small delay inreproduced signals. Thus, when applying to the teleconferencing systemor the like, natural conversation can be conducted. The calculation ofrespectively deriving the transfer functions of said two or four audiotransfer systems using the set of said two uncorrelated compositesignals as the reference signals may be, for example, a calculation ofrespectively deriving transfer functions of said two or four audiotransfer systems based on a cross-spectrum calculation between said twouncorrelated composite signals and the respective microphone collectedaudio signals. Further, the calculation of respectively deriving thetransfer functions of said two or four audio transfer systems based onsaid cross-spectrum calculation may be, for example, a calculation ofderiving cross spectra between said two uncorrelated composite signalsand the respective microphone collected audio signals, andensemble-averaging them in a predetermined time period per crossspectrum derive a plurality of kinds of composite transfer functionsobtained by combining transfer functions of a plurality of suitablesystems among said two or four audio transfer systems, thereby to derivetransfer functions of said two or four audio transfer systems based onsaid plurality of kinds of composite transfer functions.

[0023] A stereo echo cancel method of this invention is a methodwherein, with respect to a space provided therein with two loudspeakersand one or two microphones and forming two or four audio transfersystems in which stereo sounds reproduced by said respectiveloudspeakers are collected by said microphones, transfer functions ofsaid two or four audio transfer systems are estimated so as to setcorresponding filter characteristics, respectively, echo cancel signalsare respectively produced by giving said filter characteristics tocorresponding signals to be reproduced by said respective loudspeakers,and said echo cancel signals are subtracted from corresponding collectedaudio signals of said one or two microphones, thereby performing echocancellation, and wherein a principal component analysis is applied tosaid stereo audio signals to produce two uncorrelated composite signalsthat are orthogonal to each other, and estimated errors of transferfunctions of said two or four audio transfer systems are respectivelyderived using a set of said two uncorrelated composite signals asreference signals, thereby to update corresponding filtercharacteristics to values that cancel said estimated errors. Accordingto this invention, since the mutually orthogonal two signals produced byapplying the principal component analysis to the stereo audio signalsare uncorrelated with each other, the estimated errors of the transferfunctions of said two or four audio transfer systems are respectivelyderived using such two signals so as to successively update thecorresponding filter characteristics to the values that cancel suchestimated errors, thereby to enable echo cancellation. In accordancetherewith, since the stereo signals can be reproduced from theloudspeakers with no or less processing, which induces deterioration,applied to the stereo signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. It is also possible to update theecho cancel coefficients (filter characteristics) in real time. Thefilter characteristics can be updated, for example, per suitablydetermined prescribed time period (e.g. time period of performing saidensemble averaging). The calculation of respectively deriving theestimated errors of the transfer functions of said two or four audiotransfer systems using the set of said two uncorrelated compositesignals as the reference signals may be, for example, a calculation ofrespectively deriving estimated errors of transfer functions of said twoor four audio transfer systems based on a cross-spectrum calculationbetween said two uncorrelated composite signals and respective echocancel error signals obtained by subtracting the corresponding echocancel signals from the collected audio signals of said one or twomicrophones. Further, the calculation of respectively deriving theestimated errors of the transfer functions of said two or four audiotransfer systems based on the cross-spectrum calculation between saidtwo uncorrelated composite signals and said echo cancel error signals,may be, for example, a calculation of deriving cross spectra betweensaid two uncorrelated composite signals and said echo cancel errorsignals, and ensemble-averaging them in a predetermined time period percross spectrum to derive a plurality of kinds of transfer functioncomposite estimated errors obtained by combining estimated errors oftransfer functions of a plurality of suitable systems among said two orfour audio transfer systems, thereby to derive estimated errors of thetransfer functions of said two or four audio transfer systems based onsaid plurality of kinds of transfer function composite estimated errors.In this case, it may be arranged that double talk in which sounds otherthan those reproduced by said loudspeakers are inputted into saidmicrophones is detected and, when the double talk is detected, an updateperiod of said filter characteristics is made relatively longer,whereas, when the double talk is not detected, the update period of saidfilter characteristics is made relatively shorter, so that it ispossible to fully converge the estimated errors when the double talkexists, and further, quicken the convergence of the estimated errorswhen there is no double talk.

[0024] A stereo audio transmission method of this invention is suchthat, with respect to two spaces each forming said four audio transfersystems, any of the foregoing multi-channel echo cancel methods iscarried out, so that the stereo audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the stereo audio transmission withreduced echo cancellation can be performed between two spots, which, forexample, can be applied to the teleconferencing system or the like.

[0025] A stereo echo canceller of this invention is a stereo echocanceller wherein, with respect to a space provided therein with twoloudspeakers and two microphones and forming four audio transfer systemsin which stereo sounds reproduced by said respective loudspeakers arecollected by said respective microphones, an audio signal supplied tothe first loudspeaker is subjected to convolution calculations by firstand second filter means, respectively, which are provided correspondingto the first and second microphones, so as to produce first and secondecho cancel signals, an audio signal supplied to the second loudspeakeris subjected to convolution calculations by third and fourth filtermeans, respectively, which are provided corresponding to the first andsecond microphones, so as to produce third and fourth echo cancelsignals, echo cancellation is performed by subtracting, using firstsubtracting means, said first and third echo cancel signals from acollected audio signal of the first microphone, and echo cancellation isperformed by subtracting, using second subtracting means, said secondand fourth echo cancel signals from a collected audio signal of thesecond microphone, said stereo echo canceller comprising: transferfunction calculating means for respectively deriving filtercharacteristics corresponding to transfer functions of said four audiotransfer systems based on a cross-spectrum calculation between a sumsignal and a difference signal of stereo audio signals to be reproducedby said respective loudspeakers and the collected audio signals of saidrespective microphones, thereby to set said derived filtercharacteristics to corresponding ones of said first to fourth filtermeans, respectively. It may be arranged, for example, that the stereoecho canceller of this invention comprises input means for inputtingsaid stereo audio signals; sum/difference signal producing means forproducing a sum signal and a difference signal of the stereo audiosignals inputted from said input means; and a main signal transmissionsystem for transmitting the stereo audio signals inputted from saidinput means to said respective loudspeakers not through saidsum/difference signal producing means, wherein said transfer functioncalculating means derives the filter characteristics corresponding tothe transfer functions of said four audio transfer systems based on thecross-spectrum calculation between the sum signal and the differencesignal produced by said sum/difference signal producing means and therespective microphone collected audio signals, and sets the derivedfilter characteristics to corresponding ones of said first to fourthfilter means, respectively. Alternatively, it may be arranged that thestereo echo canceller comprises input means for inputting said stereoaudio signals; sum/difference signal producing means for producing a sumsignal and a difference signal of the stereo audio signals inputted fromsaid input means; and stereo audio signal demodulating means forcalculating the sum of and a difference between the sum signal and thedifference signal produced by said sum/difference signal producing meansso as to recover the original stereo audio signals, wherein the stereoaudio signals recovered by said stereo audio signal demodulating meansis transmitted to said respective loudspeakers, and said transferfunction calculating means derives the filter characteristicscorresponding to the transfer functions of said four audio transfersystems based on the cross-spectrum calculation between the sum signaland the difference signal produced by said sum/difference signalproducing means and the respective microphone collected audio signals,and sets the derived filter characteristics to corresponding ones ofsaid first to fourth filter means, respectively. Alternatively, it mayalso be arranged that the stereo echo canceller comprises input meansfor inputting a sum signal and a difference signal of said stereo audiosignals; and stereo audio signal demodulating means for calculating thesum of and a difference between said inputted sum signal and differencesignal so as to recover the original stereo audio signals, wherein thestereo audio signals recovered by said stereo audio signal demodulatingmeans is transmitted to said respective loudspeakers, and said transferfunction calculating means derives the filter characteristicscorresponding to the transfer functions of said four audio transfersystems based on the cross-spectrum calculation between said inputtedsum signal and difference signal and the respective microphone collectedaudio signals, and sets the derived filter characteristics tocorresponding ones of said first to fourth filter means, respectively.

[0026] A stereo echo canceller of this invention is a stereo echocanceller wherein, with respect to a space provided therein with twoloudspeakers and two microphones and forming four audio transfer systemsin which stereo sounds reproduced by said respective loudspeakers arecollected by said respective microphones, an audio signal supplied tothe first loudspeaker is subjected to convolution calculations by firstand second filter means, respectively, which are provided correspondingto the first and second microphones, so as to produce first and secondecho cancel signals, an audio signal supplied to the second loudspeakeris subjected to convolution calculations by third and fourth filtermeans, respectively, which are provided corresponding to the first andsecond microphones, so as to produce third and fourth echo cancelsignals, echo cancellation is performed by subtracting, using firstsubtracting means, said first and third echo cancel signals from acollected audio signal of the first microphone, and echo cancellation isperformed by subtracting, using second subtracting means, said secondand fourth echo cancel signals from a collected audio signal of thesecond microphone, said stereo echo canceller comprising: transferfunction calculating means for respectively deriving estimated errors oftransfer functions of said four audio transfer systems based on across-spectrum calculation between a sum signal and a difference signalof stereo audio signals to be reproduced by said respective loudspeakersand respective echo cancel error signals obtained by subtracting thecorresponding echo cancel signals from the collected audio signals ofsaid two microphones, thereby to update filter characteristics of saidfirst to fourth filter means to values that cancel said estimatederrors, respectively. It may be arranged, for example, that the stereoecho canceller of this invention comprises input means for inputtingsaid stereo audio signals; sum/difference signal producing means forproducing a sum signal and a difference signal of the stereo audiosignals inputted from said input means; and a main signal transmissionsystem for transmitting the stereo audio signals inputted from saidinput means to said respective loudspeakers not through saidsum/difference signal producing means, wherein said transfer functioncalculating means derives the estimated errors of the transfer functionsof said four audio transfer systems based on the cross-spectrumcalculation between the sum signal and the difference signal produced bysaid sum/difference signal producing means and the respective echocancel error signals, and updates the filter characteristics of saidfirst to fourth filter means to the values that cancel said estimatederrors, respectively. Alternatively, it may be arranged that the stereoecho canceller comprises input means for inputting said stereo audiosignals; sum/difference signal producing means for producing a sumsignal and a difference signal of the stereo audio signals inputted fromsaid input means; and stereo audio signal demodulating means forcalculating the sum of and a difference between the sum signal and thedifference signal produced by said sum/difference signal producing meansso as to recover the original stereo audio signals, wherein the stereoaudio signals recovered by said stereo audio signal demodulating meansis transmitted to said respective loudspeakers, and said transferfunction calculating means derives the estimated errors of the transferfunctions of said four audio transfer systems based on thecross-spectrum calculation between the sum signal and the differencesignal produced by said sum/difference signal producing means and therespective echo cancel error signals, and updates the filtercharacteristics of said first to fourth filter means to the values thatcancel said estimated errors, respectively. Alternatively, it may alsobe arranged that the stereo echo canceller comprises input means forinputting a sum signal and a difference signal of said stereo audiosignals; and stereo audio signal demodulating means for calculating thesum of and a difference between said inputted sum signal and differencesignal so as to recover the original stereo audio signals, wherein thestereo audio signals recovered by said stereo audio signal demodulatingmeans is transmitted to said respective loudspeakers, and said transferfunction calculating means derives the estimated errors of the transferfunctions of said four audio transfer systems based on thecross-spectrum calculation between said inputted sum signal anddifference signal produced by said sum/difference signal producing meansand the respective echo cancel error signals, and updates the filtercharacteristics of said first to fourth filter means to the values thatcancel said estimated errors, respectively.

[0027] A stereo echo canceller of this invention is a stereo echocanceller wherein, with respect to a space provided therein with twoloudspeakers and two microphones and forming four audio transfer systemsin which stereo sounds reproduced by said respective loudspeakers arecollected by said respective microphones, an audio signal supplied tothe first loudspeaker is subjected to convolution calculations by firstand second filter means, respectively, which are provided correspondingto the first and second microphones, so as to produce first and secondecho cancel signals, an audio signal supplied to the second loudspeakeris subjected to convolution calculations by third and fourth filtermeans, respectively, which are provided corresponding to the first andsecond microphones, so as to produce third and fourth echo cancelsignals, echo cancellation is performed by subtracting, using firstsubtracting means, said first and third echo cancel signals from acollected audio signal of the first microphone, and echo cancellation isperformed by subtracting, using second subtracting means, said secondand fourth echo cancel signals from a collected audio signal of thesecond microphone, said stereo echo canceller comprising: transferfunction calculating means for respectively deriving filtercharacteristics corresponding to transfer functions of said four audiotransfer systems based on a cross-spectrum calculation between mutuallyorthogonal two uncorrelated composite signals produced by applying aprincipal component analysis to stereo audio signals to be reproduced bysaid respective loudspeakers and the respective microphone collectedaudio signals, thereby to set said derived filter characteristics tocorresponding ones of said first to fourth filter means, respectively.It may be arranged, for example, that the stereo echo canceller of thisinvention comprises input means for inputting said stereo audio signals;orthogonalizing means for applying a principal component analysis to thestereo audio signals inputted from said input means to produce mutuallyorthogonal two uncorrelated composite signals; and a main signaltransmission system for transmitting the stereo audio signals inputtedfrom said input means to said respective loudspeakers not through saidorthogonalizing means, wherein said transfer function calculating meansderives the filter characteristics corresponding to the transferfunctions of said four audio transfer systems based on thecross-spectrum calculation between the two uncorrelated compositesignals produced by said orthogonalizing means and the respectivemicrophone collected audio signals, and sets the derived filtercharacteristics to corresponding ones of said first to fourth filtermeans, respectively.

[0028] A stereo echo canceller of this invention is a stereo echocanceller wherein, with respect to a space provided therein with twoloudspeakers and two microphones and forming four audio transfer systemsin which stereo sounds reproduced by said respective loudspeakers arecollected by said respective microphones, an audio signal supplied tothe first loudspeaker is subjected to convolution calculations by firstand second filter means, respectively, which are provided correspondingto the first and second microphones, so as to produce first and secondecho cancel signals, an audio signal supplied to the second loudspeakeris subjected to convolution calculations by third and fourth filtermeans, respectively, which are provided corresponding to the first andsecond microphones, so as to produce third and fourth echo cancelsignals, echo cancellation is performed by subtracting, using firstsubtracting means, said first and third echo cancel signals from acollected audio signal of the first microphone, and echo cancellation isperformed by subtracting, using second subtracting means, said secondand fourth echo cancel signals from a collected audio signal of thesecond microphone, said stereo echo canceller comprising: transferfunction calculating means for respectively deriving estimated errors oftransfer functions of said four audio transfer systems based on across-spectrum calculation between mutually orthogonal two uncorrelatedcomposite signals produced by applying a principal component analysis tostereo audio signals to be reproduced by said respective loudspeakersand respective echo cancel error signals obtained by subtracting thecorresponding echo cancel signals from the collected audio signals ofsaid two microphones, thereby to update filter characteristics of saidfirst to fourth filter means to values that cancel said estimatederrors, respectively. It may be arranged, for example, that the stereoecho canceller of this invention comprises input means for inputtingsaid stereo audio signals; orthogonalizing means for applying aprincipal component analysis to the stereo audio signals inputted fromsaid input means to produce mutually orthogonal two uncorrelatedcomposite signals; and a main signal transmission system fortransmitting the stereo audio signals inputted from said input means tosaid respective loudspeakers not through said orthogonalizing means,wherein said transfer function calculating means derives the estimatederrors of the transfer functions of said four audio transfer systemsbased on the cross-spectrum calculation between the two uncorrelatedcomposite signals produced by said orthogonalizing means and therespective echo cancel error signals, and updates the filtercharacteristics of said first to fourth filter means to the values thatcancel said estimated errors, respectively. In this case, it may bearranged that double talk detecting means is provided for detectingdouble talk in which sounds other than those reproduced by saidloudspeakers are inputted into said microphones and, when the doubletalk is detected, said transfer function calculating means makesrelatively longer an update period of said filter characteristics,whereas, when the double talk is not detected, it makes relativelyshorter the update period of said filter characteristics, so that it ispossible to fully converge the estimated errors when the double talkexists, and further, quicken the convergence of the estimated errorswhen there is no double talk.

[0029] The stereo echo canceller of this invention may be furtherprovided with correlation detecting means for detecting the correlationbetween the sum signal and the difference signal of said stereo audiosignals and, when a value of said correlation is no less than aprescribed value, stopping updating of said filter characteristics,thereby to prevent the echo cancel error signals from unexpectedlyincreasing.

[0030] A stereo sound transfer apparatus of this invention is such that,with respect to two spaces each forming said four audio transfersystems, any of said stereo echo cancellers is arranged in each space,so that the stereo audio signals, which have been echo-canceled by saidstereo echo cancellers, are transmitted between said two spaces.

[0031] [Inventions of Claims 22 to 27 and Inventions Relating to SuchInventions]

[0032] A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsreproduced by said respective loudspeakers and having a correlation witheach other are collected by said microphones, composite transferfunctions of said plurality of audio transfer systems are estimated soas to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to composite signals of individual signals to bereproduced by said respective loudspeakers, and said echo cancel signalsare subtracted from individual collected audio signals of said one orplurality of microphones, thereby performing echo cancellation, andwherein, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals (e.g. suitably combining said multi-channel audio signalsto produce a plurality of low-correlation composite signals having alower correlation with each other than that between said multi-channelaudio signals and using a set of said plurality of low-correlationcomposite signals as reference signals, or directly inputting a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),composite transfer functions of said plurality of audio transfer systemsare respectively derived, thereby to set corresponding filtercharacteristics. According to this invention, using as reference signalsa set of a plurality of low-correlation composite signals whichcorrespond to signals obtained by suitably combining multi-channel audiosignals having a correlation therebetween and which have a lowercorrelation with each other than that between such multi-channel audiosignals, the composite transfer functions of said plurality of audiotransfer systems are respectively derived, and corresponding filtercharacteristics are set, thereby to enable echo cancellation. Inaccordance therewith, since the multi-channel audio signals can bereproduced from the loudspeakers with no or less processing, whichinduces deterioration, applied to the multi-channel audio signals,excellent reproduced tone quality can be achieved. Further, there is noor only a small delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. The calculation of respectively deriving the compositetransfer functions of said plurality of audio transfer systems using asthe reference signals the set of the plurality of low-correlationcomposite signals, may be, for example, a calculation of respectivelyderiving the composite transfer functions of said plurality of audiotransfer systems based on a cross-spectrum calculation between theplurality of low-correlation composite signals and the individualcollected audio signals of the respective microphones. Further, thecalculation of respectively deriving the composite transfer functions ofsaid plurality of audio transfer systems based on said cross-spectrumcalculation, may be, for example, a calculation of combining saidmulti-channel audio signals through addition or subtraction to produce aplurality of low-correlation composite signals having a lowercorrelation with each other than that between said multi-channel audiosignals, deriving cross spectra between said plurality oflow-correlation composite signals and the individual collected audiosignals of the respective microphones, and ensemble-averaging them in apredetermined time period per cross spectrum to derive compositetransfer functions of said plurality of audio transfer systems.

[0033] A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsreproduced by said respective loudspeakers and having a correlation witheach other are collected by said microphones, composite transferfunctions of said plurality of audio transfer systems are estimated soas to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to composite signals of individual signals to bereproduced by said respective loudspeakers, and said echo cancel signalsare subtracted from individual collected audio signals of said one orplurality of microphones, thereby performing echo cancellation, andwherein, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals (e.g. suitably combining said multi-channel audio signalsto produce a plurality of low-correlation composite signals having alower correlation with each other than that between said multi-channelaudio signals and using a set of said plurality of low-correlationcomposite signals as reference signals, or directly inputting a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),estimated errors of composite transfer functions of said plurality ofaudio transfer systems are respectively derived, thereby to updatecorresponding filter characteristics to values that cancel saidestimated errors. According to this invention, using as referencesignals a set of a plurality of low-correlation composite signals whichcorrespond to signals obtained by suitably combining multi-channel audiosignals having a correlation therebetween and which have a lowercorrelation with each other than that between such multi-channel audiosignals, the estimated errors of the composite transfer functions ofsaid plurality of audio transfer systems are respectively derived so asto successively update the corresponding filter characteristics to thevalues that cancel said estimated errors, thereby to enable echocancellation. In accordance therewith, since the multi-channel audiosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the multi-channelaudio signals, excellent reproduced tone quality can be achieved.Further, there is no or only a small delay in reproduced signals. Thus,when applying to the teleconferencing system or the like, naturalconversation can be conducted. It is also possible to update the filtercharacteristics in real time. The calculation of respectively derivingthe estimated errors of the composite transfer functions of saidplurality of audio transfer systems using the set of the plurality oflow-correlation composite signals as the reference signals, may be, forexample, a calculation of respectively deriving the estimated errors ofthe composite transfer functions of said plurality of audio transfersystems based on a cross-spectrum calculation between said plurality oflow-correlation composite signals and echo cancel error signals obtainedby subtracting the corresponding echo cancel signals from the individualcollected audio signals of said one or plurality of microphones.Further, the calculation of respectively deriving the estimated errorsof the composite transfer functions of said plurality of audio transfersystems based on said cross-spectrum calculation, may be, for example, acalculation of combining said multi-channel audio signals throughaddition or subtraction to produce a plurality of low-correlationcomposite signals having a lower correlation with each other than thatbetween said multi-channel audio signals, deriving cross spectra betweensaid plurality of low-correlation composite signals and the echo cancelerror signals obtained by subtracting the corresponding echo cancelsignals from the individual collected audio signals of said one orplurality of microphones, and ensemble-averaging them in a predeterminedtime period per cross spectrum to derive estimated errors of thecomposite transfer functions of said plurality of audio transfersystems. Further, the correlation between said plurality oflow-correlation composite signals is detected and, when a value of saidcorrelation is no less than a prescribed value, updating of said filtercharacteristics is stopped, thereby to prevent the echo cancel errorsignals from unexpectedly increasing.

[0034] A multi-channel sound transfer method of this invention is suchthat, with respect to two spaces each forming said plurality of audiotransfer systems, any of the foregoing multi-channel echo cancel methodsis carried out, so that the multi-channel audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the multi-channel audiotransmission with reduced echo cancellation can be performed between twospots, which, for example, can be applied to the teleconferencing systemor the like.

[0035] A stereo echo cancel method of this invention is a methodwherein, with respect to a space provided therein with two loudspeakersand one or two microphones and forming two or four audio transfersystems in which stereo sounds reproduced by said respectiveloudspeakers are collected by said microphones, composite transferfunctions of said two or four audio transfer systems are estimated so asto set corresponding filter characteristics, respectively, echo cancelsignals are respectively produced by giving said set filtercharacteristics to composite signals of individual signals to bereproduced by said respective loudspeakers, and said echo cancel signalsare subtracted from individual collected audio signals of said one ortwo microphones, thereby performing echo cancellation, and wherein,using a sum signal and a difference signal of said stereo audio signalsas reference signals, composite transfer functions of said two or fouraudio transfer systems are respectively derived, thereby to setcorresponding filter characteristics. According to this invention, sincethe sum signal and the difference signal of the stereo audio signalshave a low correlation therebetween, the composite transfer functions ofthe two or four audio transfer systems are respectively derived usingthe sum signal and the difference signal as reference signals, so as toset the corresponding filter characteristics, thereby to enable echocancellation. In accordance therewith, since the stereo signals can bereproduced from the loudspeakers with no or less processing, whichinduces deterioration, applied to the stereo signals, excellentreproduced tone quality can be achieved. Further, there is no or only asmall delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. The calculation of respectively deriving the compositetransfer functions of said two or four audio transfer systems using thesum signal and the difference signal of said stereo audio signals as thereference signals, may be, for example, a calculation of respectivelyderiving the composite transfer functions of said two or four audiotransfer systems based on a cross-spectrum calculation between the sumsignal and the difference signal, and the individual collected audiosignals of the respective microphones. Further, the calculation ofrespectively deriving the composite transfer functions of said two orfour audio transfer systems based on said cross-spectrum calculation,may be, for example, a calculation of deriving cross spectra between thesum signal and the difference signal of said stereo audio signals andthe individual collected audio signals of the respective microphones,and ensemble-averaging them in a predetermined time period per crossspectrum to derive composite transfer functions of said two or fouraudio transfer systems.

[0036] A stereo echo cancel method of this invention is a methodwherein, with respect to a space provided therein with two loudspeakersand one or two microphones and forming two or four audio transfersystems in which stereo sounds reproduced by said respectiveloudspeakers are collected by said microphones, composite transferfunctions of said two or four audio transfer systems are estimated so asto set corresponding filter characteristics, respectively, echo cancelsignals are respectively produced by giving said set filtercharacteristics to composite signals of individual signals to bereproduced by said respective loudspeakers, and said echo cancel signalsare subtracted from individual collected audio signals of said one ortwo microphones, thereby performing echo cancellation, and wherein,using a sum signal and a difference signal of said stereo audio signalsas reference signals, estimated errors of composite transfer functionsof said two or four audio transfer systems are respectively derived,thereby to update corresponding filter characteristics to values thatcancel said estimated errors. According to this invention, since the sumsignal and the difference signal of the stereo audio signals have a lowcorrelation therebetween, the estimated errors of the composite transferfunctions of the two or four audio transfer systems are respectivelyderived using the sum signal and the difference signal as referencesignals, so as to successively update the corresponding filtercharacteristics to the values that cancel the estimated errors, therebyto enable echo cancellation. In accordance therewith, since the stereosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the stereo signals,excellent reproduced tone quality can be achieved. Further, there is noor only a small delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. The calculation of respectively deriving the estimated errorsof the composite transfer functions of said two or four audio transfersystems using the sum signal and the difference signal of said stereoaudio signals as the reference signals, may be, for example, acalculation of respectively deriving the estimated errors of thecomposite transfer functions of said two or four audio transfer systemsbased on a cross-spectrum calculation between the sum signal and thedifference signal of said stereo audio signals and respective echocancel error signals obtained by subtracting the corresponding echocancel signals from the individual collected audio signals of said oneor two microphones. Further, the calculation of respectively derivingthe estimated errors of the composite transfer functions of said two orfour audio transfer systems based on the cross-spectrum calculationbetween the sum signal and the difference signal of said stereo audiosignals and said echo cancel error signals, may be, for example, acalculation of deriving cross spectra between the sum signal and thedifference signal of said stereo audio signals and said echo cancelerror signals, and ensemble-averaging them in a predetermined timeperiod per cross spectrum to derive estimated errors of the compositetransfer functions of said two or four audio transfer systems. Further,the correlation between the sum signal and the difference signal of saidstereo audio signals is detected and, when a value of said correlationis no less than a prescribed value, updating of said filtercharacteristics is stopped, thereby to prevent the echo cancel errorsignals from unexpectedly increasing.

[0037] A stereo audio transmission method of this invention is suchthat, with respect to two spaces each forming said four audio transfersystems, any of the foregoing multi-channel echo cancel methods iscarried out, so that the stereo audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the stereo audio transmission withreduced echo cancellation can be performed between two spots, which, forexample, can be applied to the teleconferencing system or the like.

[0038] A stereo echo canceller of this invention is a stereo echocanceller wherein, with respect to a space provided therein with twoloudspeakers and two microphones and forming four audio transfer systemsin which stereo sounds reproduced by said respective loudspeakers arecollected by said respective microphones, a sum signal of stereo audiosignals to be reproduced by said respective loudspeakers is subjected toconvolution calculations by first and second filter means, respectively,so as to produce first and second echo cancel signals, a differencesignal of the stereo audio signals to be reproduced by said respectiveloudspeakers is subjected to convolution calculations by third andfourth filter means, respectively, so as to produce third and fourthecho cancel signals, echo cancellation is performed by subtracting,using first subtracting means, said first and third echo cancel signalsfrom a collected audio signal of the first microphone, and echocancellation is performed by subtracting, using second subtractingmeans, said second and fourth echo cancel signals from a collected audiosignal of the second microphone, said stereo echo canceller comprising:transfer function calculating means for respectively deriving filtercharacteristics corresponding to composite transfer functions of saidfour audio transfer systems based on a cross-spectrum calculationbetween the sum signal and the difference signal of the stereo audiosignals to be reproduced by said respective loudspeakers and therespective microphone collected audio signals, thereby to set saidderived filter characteristics to corresponding ones of said first tofourth filter means, respectively. It may be arranged, for example, thatthe stereo echo canceller of this invention comprises input means forinputting said stereo audio signals; sum/difference signal producingmeans for producing a sum signal and a difference signal of the stereoaudio signals inputted from said input means; and a main signaltransmission system for transmitting the stereo audio signals inputtedfrom said input means to said respective loudspeakers not through saidsum/difference signal producing means, wherein said transfer functioncalculating means derives the filter characteristics corresponding tothe composite transfer functions of said four audio transfer systemsbased on the cross-spectrum calculation between the sum signal and thedifference signal produced by said sum/difference signal producing meansand the respective microphone collected audio signals, and sets thederived filter characteristics to corresponding ones of said first tofourth filter means, respectively.

[0039] A stereo echo canceller of this invention is a stereo echocanceller wherein, with respect to a space provided therein with twoloudspeakers and two microphones and forming four audio transfer systemsin which stereo sounds reproduced by said respective loudspeakers arecollected by said respective microphones, a sum signal of stereo audiosignals to be reproduced by said respective loudspeakers is subjected toconvolution calculations by first and second filter means, respectively,so as to produce first and second echo cancel signals, a differencesignal of the stereo audio signals to be reproduced by said respectiveloudspeakers is subjected to convolution calculations by third andfourth filter means, respectively, so as to produce third and fourthecho cancel signals, echo cancellation is performed by subtracting,using first subtracting means, said first and third echo cancel signalsfrom a collected audio signal of the first microphone, and echocancellation is performed by subtracting, using second subtractingmeans, said second and fourth echo cancel signals from a collected audiosignal of the second microphone, said stereo echo canceller comprising:transfer function calculating means for respectively deriving estimatederrors of composite transfer functions of said four audio transfersystems based on a cross-spectrum calculation between the sum signal andthe difference signal of the stereo audio signals to be reproduced bysaid respective loudspeakers and respective echo cancel error signalsobtained by subtracting the corresponding echo cancel signals from thecollected audio signals of said two microphones, thereby to updatefilter characteristics of said first to fourth filter means to valuesthat cancel said estimated errors, respectively. It may be arranged, forexample, that the stereo echo canceller of this invention comprisesinput means for inputting said stereo audio signals; sum/differencesignal producing means for producing a sum signal and a differencesignal of the stereo audio signals inputted from said input means; and amain signal transmission system for transmitting the stereo audiosignals inputted from said input means to said respective loudspeakersnot through said sum/difference signal producing means, wherein saidtransfer function calculating means derives the estimated errors of thecomposite transfer functions of said four audio transfer systems basedon the cross-spectrum calculation between the sum signal and thedifference signal produced by said sum/difference signal producing meansand the respective echo cancel error signals, and updates the filtercharacteristics of said first to fourth filter means to the values thatcancel said estimated errors, respectively.

[0040] The stereo echo canceller of this invention may be furtherprovided with correlation detecting means for detecting the correlationbetween the sum signal and the difference signal of said stereo audiosignals and, when a value of said correlation is no less than aprescribed value, stopping updating of said filter characteristics,thereby to prevent the echo cancel error signals from unexpectedlyincreasing.

[0041] A stereo sound transfer apparatus of this invention is such that,with respect to two spaces each forming said four audio transfersystems, any of said stereo echo cancellers is arranged in each space,so that the stereo audio signals, which have been echo-canceled by saidstereo echo cancellers, are transmitted between said two spaces.

[0042] [Inventions of Claims 28 to 31 and Inventions Relating to SuchInventions]

[0043] A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsreproduced by said respective loudspeakers and having a correlation witheach other are collected by said microphones, composite transferfunctions of said plurality of audio transfer systems are estimated soas to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to individual signals to be reproduced by saidrespective loudspeakers, and said echo cancel signals are subtractedfrom composite signals of individual collected audio signals of said oneor plurality of microphones, thereby performing echo cancellation, andwherein, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals (e.g. suitably combining said multi-channel audio signalsto produce a plurality of low-correlation composite signals having alower correlation with each other than that between said multi-channelaudio signals and using a set of said plurality of low-correlationcomposite signals as reference signals, or directly inputting a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),composite transfer functions of said plurality of audio transfer systemsare respectively derived, thereby to set corresponding filtercharacteristics. According to this invention, using as reference signalsa set of a plurality of low-correlation composite signals whichcorrespond to signals obtained by suitably combining multi-channel audiosignals having a correlation therebetween and which have a lowercorrelation with each other than that between such multi-channel audiosignals, the composite transfer functions of said plurality of audiotransfer systems are respectively derived, and corresponding filtercharacteristics are set, thereby to enable echo cancellation. Inaccordance therewith, since the multi-channel audio signals can bereproduced from the loudspeakers with no or less processing, whichinduces deterioration, applied to the multi-channel audio signals,excellent reproduced tone quality can be achieved. Further, there is noor only a small delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. The calculation of respectively deriving the compositetransfer functions of said plurality of audio transfer systems using asthe reference signals the set of the plurality of low-correlationcomposite signals, may be, for example, a calculation of respectivelyderiving the composite transfer functions of said plurality of audiotransfer systems based on a cross-spectrum calculation between theplurality of low-correlation composite signals and the composite signalsof the individual collected audio signals of the respective microphones.Further, the calculation of respectively deriving the composite transferfunctions of said plurality of audio transfer systems based on saidcross-spectrum calculation, may be, for example, a calculation ofcombining said multi-channel audio signals through addition orsubtraction to produce a plurality of low-correlation composite signalshaving a lower correlation with each other than that between saidmulti-channel audio signals, deriving cross spectra between saidplurality of low-correlation composite signals and the composite signalsof the individual collected audio signals of the respective microphones,and ensemble-averaging them in a predetermined time period per crossspectrum to derive composite transfer functions of said plurality ofaudio transfer systems.

[0044] A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsreproduced by said respective loudspeakers and having a correlation witheach other are collected by said microphones, composite transferfunctions of said plurality of audio transfer systems are estimated soas to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to individual signals to be reproduced by saidrespective loudspeakers, and said echo cancel signals are subtractedfrom composite signals of individual collected audio signals of said oneor plurality of microphones, thereby performing echo cancellation, andwherein, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals (e.g. suitably combining said multi-channel audio signalsto produce a plurality of low-correlation composite signals having alower correlation with each other than that between said multi-channelaudio signals and using a set of said plurality of low-correlationcomposite signals as reference signals, or directly inputting a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals and using the set of said plurality oflow-correlation composite signals as reference signals, or the like),estimated errors of composite transfer functions of said plurality ofaudio transfer systems are respectively derived, thereby to updatecorresponding filter characteristics to values that cancel saidestimated errors. According to this invention, using as referencesignals a set of a plurality of low-correlation composite signals whichcorrespond to signals obtained by suitably combining multi-channel audiosignals having a correlation therebetween and which have a lowercorrelation with each other than that between such multi-channel audiosignals, the estimated errors of the composite transfer functions ofsaid plurality of audio transfer systems are respectively derived so asto successively update the corresponding filter characteristics to thevalues that cancel said estimated errors, thereby to enable echocancellation. In accordance therewith, since the multi-channel audiosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the multi-channelaudio signals, excellent reproduced tone quality can be achieved.Further, there is no or only a small delay in reproduced signals. Thus,when applying to the teleconferencing system or the like, naturalconversation can be conducted. It is also possible to update the filtercharacteristics in real time. The calculation of respectively derivingthe estimated errors of the composite transfer functions of saidplurality of audio transfer systems using the set of the plurality oflow-correlation composite signals as the reference signals, may be, forexample, a calculation of respectively deriving the estimated errors ofthe composite transfer functions of said plurality of audio transfersystems based on a cross-spectrum calculation between said plurality oflow-correlation composite signals and echo cancel error signals obtainedby subtracting the corresponding echo cancel signals from the compositesignals of the individual collected audio signals of said one orplurality of microphones. Further, the calculation of respectivelyderiving the estimated errors of the composite transfer functions ofsaid plurality of audio transfer systems based on said cross-spectrumcalculation, may be, for example, a calculation of combining saidmulti-channel audio signals through addition or subtraction to produce aplurality of low-correlation composite signals having a lowercorrelation with each other than that between said multi-channel audiosignals, deriving cross spectra between said plurality oflow-correlation composite signals and the echo cancel error signalsobtained by subtracting the corresponding echo cancel signals from thecomposite signals of the individual collected audio signals of said oneor plurality of microphones, and ensemble-averaging them in apredetermined time period per cross spectrum to derive estimated errorsof the composite transfer functions of said plurality of audio transfersystems. Further, the correlation between said plurality oflow-correlation composite signals is detected and, when a value of saidcorrelation is no less than a prescribed value, updating of said filtercharacteristics is stopped, thereby to prevent the echo cancel errorsignals from unexpectedly increasing.

[0045] A multi-channel sound transfer method of this invention is suchthat, with respect to two spaces each forming said plurality of audiotransfer systems, any of the foregoing multi-channel echo cancel methodsis carried out, so that the multi-channel audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the multi-channel audiotransmission with reduced echo cancellation can be performed between twospots, which, for example, can be applied to the teleconferencing systemor the like.

[0046] A stereo echo cancel method of this invention is a methodwherein, with respect to a space provided therein with two loudspeakersand one or two microphones and forming two or four audio transfersystems in which stereo sounds reproduced by said respectiveloudspeakers are collected by said microphones, composite transferfunctions of said two or four audio transfer systems are estimated so asto set corresponding filter characteristics, respectively, echo cancelsignals are respectively produced by giving said set filtercharacteristics to individual signals to be reproduced by saidrespective loudspeakers, and said echo cancel signals are subtractedfrom composite signals of individual collected audio signals of said oneor two microphones, thereby performing echo cancellation, and wherein,using a sum signal and a difference signal of said stereo audio signalsas reference signals, composite transfer functions of said two or fouraudio transfer systems are respectively derived, thereby to setcorresponding filter characteristics. According to this invention, sincethe sum signal and the difference signal of the stereo audio signalshave a low correlation therebetween, estimated errors of the compositetransfer functions of the two or four audio transfer systems arerespectively derived using the sum signal and the difference signal asreference signals, so as to successively update the corresponding filtercharacteristics to values that cancel said estimated errors, thereby toenable echo cancellation. In accordance therewith, since the stereosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the stereo signals,excellent reproduced tone quality can be achieved. Further, there is noor only a small delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. The calculation of respectively deriving the compositetransfer functions of said two or four audio transfer systems using thesum signal and the difference signal of said stereo audio signals as thereference signals, may be, for example, a calculation of respectivelyderiving the composite transfer functions of said two or four audiotransfer systems based on a cross-spectrum calculation between the sumsignal and the difference signal, and the composite signals of theindividual collected audio signals of the respective microphones.Further, the calculation of respectively deriving the composite transferfunctions of said two or four audio transfer systems based on saidcross-spectrum calculation, may be, for example, a calculation ofderiving cross spectra between the sum signal and the difference signalof said stereo audio signals and the composite signals of the individualcollected audio signals of the respective microphones, andensemble-averaging them in a predetermined time period per crossspectrum to derive composite transfer functions of said two or fouraudio transfer systems.

[0047] A stereo echo cancel method of this invention is a methodwherein, with respect to a space provided therein with two loudspeakersand one or two microphones and forming two or four audio transfersystems in which stereo sounds reproduced by said respectiveloudspeakers are collected by said microphones, composite transferfunctions of said two or four audio transfer systems are estimated so asto set corresponding filter characteristics, respectively, echo cancelsignals are respectively produced by giving said set filtercharacteristics to individual signals to be reproduced by saidrespective loudspeakers, and said echo cancel signals are subtractedfrom composite signals of individual collected audio signals of said oneor two microphones, thereby performing echo cancellation, and wherein,using a sum signal and a difference signal of said stereo audio signalsas reference signals, estimated errors of composite transfer functionsof said two or four audio transfer systems are respectively derived,thereby to update corresponding filter characteristics to values thatcancel said estimated errors. According to this invention, using asreference signals a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combiningmulti-channel audio signals having a correlation therebetween and whichhave a lower correlation with each other than that between suchmulti-channel audio signals, the estimated errors of the compositetransfer functions of said plurality of audio transfer systems arerespectively derived, so as to successively update the correspondingfilter characteristics to the values that cancel the estimated errors,thereby to enable echo cancellation. In accordance therewith, since themulti-channel audio signals can be reproduced from the loudspeakers withno or less processing, which induces deterioration, applied to themulti-channel audio signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. It is also possible to update thefilter characteristics in real time. The calculation of respectivelyderiving the estimated errors of the composite transfer functions ofsaid two or four audio transfer systems using the sum signal and thedifference signal of said stereo audio signals as the reference signals,may be, for example, a calculation of respectively deriving theestimated errors of the composite transfer functions of said two or fouraudio transfer systems based on a cross-spectrum calculation between thesum signal and the difference signal of said stereo audio signals andrespective echo cancel error signals obtained by subtracting thecorresponding echo cancel signals from the composite signals of theindividual collected audio signals of said one or two microphones.Further, the calculation of respectively deriving the estimated errorsof the composite transfer functions of said two or four audio transfersystems based on the cross-spectrum calculation between the sum signaland the difference signal of said stereo audio signals and said echocancel error signals, may be, for example, a calculation of derivingcross spectra between the sum signal and the difference signal of saidstereo audio signals and said echo cancel error signals, andensemble-averaging them in a predetermined time period per crossspectrum to derive estimated errors of the composite transfer functionsof said two or four audio transfer systems. Further, the correlationbetween the sum signal and the difference signal of said stereo audiosignals is detected and, when a value of said correlation is no lessthan a prescribed value, updating of said filter characteristics isstopped, thereby to prevent the echo cancel error signals fromunexpectedly increasing.

[0048] A stereo audio transmission method of this invention is suchthat, with respect to two spaces each forming said four audio transfersystems, any of the foregoing multi-channel echo cancel methods iscarried out, so that the stereo audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the stereo audio transmission withreduced echo cancellation can be performed between two spots, which, forexample, can be applied to the teleconferencing system or the like.

[0049] A stereo echo canceller of this invention is a stereo echocanceller wherein, with respect to a space provided therein with twoloudspeakers and two microphones and forming four audio transfer systemsin which stereo sounds reproduced by said respective loudspeakers arecollected by said respective microphones, an audio signal supplied tothe first loudspeaker is subjected to convolution calculations by firstand second filter means, respectively, so as to produce first and secondecho cancel signals, an audio signal supplied to the second loudspeakeris subjected to convolution calculations by third and fourth filtermeans, respectively, so as to produce third and fourth echo cancelsignals, echo cancellation is performed by subtracting, using firstsubtracting means, said first and third echo cancel signals from a sumsignal of collected audio signals of the respective microphones, andecho cancellation is performed by subtracting, using second subtractingmeans, said second and fourth echo cancel signals from a differencesignal of the collected audio signals of the respective microphones,said stereo echo canceller comprising: transfer function calculatingmeans for respectively deriving filter characteristics corresponding tocomposite transfer functions of said four audio transfer systems basedon a cross-spectrum calculation between a sum signal and a differencesignal of stereo audio signals to be reproduced by said respectiveloudspeakers and the sum signal and the difference signal of therespective microphone collected audio signals, thereby to set saidderived filter characteristics to corresponding ones of said first tofourth filter means, respectively.

[0050] A stereo echo canceller of this invention is a stereo echocanceller wherein, with respect to a space provided therein with twoloudspeakers and two microphones and forming four audio transfer systemsin which stereo sounds reproduced by said respective loudspeakers arecollected by said respective microphones, an audio signal supplied tothe first loudspeaker is subjected to convolution calculations by firstand second filter means, respectively, so as to produce first and secondecho cancel signals, an audio signal supplied to the second loudspeakeris subjected to convolution calculations by third and fourth filtermeans, respectively, so as to produce third and fourth echo cancelsignals, echo cancellation is performed by subtracting, using firstsubtracting means, said first and third echo cancel signals from a sumsignal of collected audio signals of the respective microphones, andecho cancellation is performed by subtracting, using second subtractingmeans, said second and fourth echo cancel signals from a differencesignal of the collected audio signals of the respective microphones,said stereo echo canceller comprising: transfer function calculatingmeans for respectively deriving estimated errors of composite transferfunctions of said four audio transfer systems based on a cross-spectrumcalculation between a sum signal and a difference signal of stereo audiosignals to be reproduced by said respective loudspeakers and respectiveecho cancel error signals obtained by subtracting the corresponding echocancel signals from the sum signal and the difference signal of therespective microphone collected audio signals, thereby to update filtercharacteristics of said first to fourth filter means to values thatcancel said estimated errors, respectively.

[0051] The stereo echo canceller of this invention may be furtherprovided with correlation detecting means for detecting the correlationbetween the sum signal and the difference signal of said stereo audiosignals and, when a value of said correlation is no less than aprescribed value, stopping updating of said filter characteristics,thereby to prevent the echo cancel error signals from unexpectedlyincreasing.

[0052] A stereo sound transfer apparatus of this invention is such that,with respect to two spaces each forming said four audio transfersystems, any of said stereo echo cancellers is arranged in each space,so that the stereo audio signals, which have been echo-canceled by saidstereo echo cancellers, are transmitted between said two spaces.

[0053] [Inventions of Claims 32 to 35 and Inventions Relating to SuchInventions]

[0054] A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsreproduced by said respective loudspeakers and having a correlation witheach other are collected by said microphones, composite transferfunctions of said plurality of audio transfer systems are estimated soas to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to composite signals of individual signals to bereproduced by said respective loudspeakers, and said echo cancel signalsare subtracted from composite signals of individual collected audiosignals of said one or plurality of microphones, thereby performing echocancellation, and wherein, using as reference signals a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals (e.g. suitably combining saidmulti-channel audio signals to produce a plurality of low-correlationcomposite signals having a lower correlation with each other than thatbetween said multi-channel audio signals and using a set of saidplurality of low-correlation composite signals as reference signals, ordirectly inputting a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combining saidmulti-channel audio signals and which have a lower correlation with eachother than that between said multi-channel audio signals and using theset of said plurality of low-correlation composite signals as referencesignals, or the like), composite transfer functions of said plurality ofaudio transfer systems are respectively derived, thereby to setcorresponding filter characteristics. According to this invention, usingas reference signals a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combiningmulti-channel audio signals having a correlation therebetween and whichhave a lower correlation with each other than that between suchmulti-channel audio signals, the composite transfer functions of saidplurality of audio transfer systems are respectively derived, andcorresponding filter characteristics are set, thereby to enable echocancellation. In accordance therewith, since the multi-channel audiosignals can be reproduced from the loudspeakers with no or lessprocessing, which induces deterioration, applied to the multi-channelaudio signals, excellent reproduced tone quality can be achieved.Further, there is no or only a small delay in reproduced signals. Thus,when applying to the teleconferencing system or the like, naturalconversation can be conducted. The calculation of respectively derivingthe composite transfer functions of said plurality of audio transfersystems using as the reference signals the set of the plurality oflow-correlation composite signals, may be, for example, a calculation ofrespectively deriving the composite transfer functions of said pluralityof audio transfer systems based on a cross-spectrum calculation betweenthe plurality of low-correlation composite signals and the compositesignals of the individual collected audio signals of the respectivemicrophones. Further, the calculation of respectively deriving thecomposite transfer functions of said plurality of audio transfer systemsbased on said cross-spectrum calculation, may be, for example, acalculation of combining said multi-channel audio signals throughaddition or subtraction to produce a plurality of low-correlationcomposite signals having a lower correlation with each other than thatbetween said multi-channel audio signals, deriving cross spectra betweensaid plurality of low-correlation composite signals and the compositesignals of the individual collected audio signals of the respectivemicrophones, and ensemble-averaging them in a predetermined time periodper cross spectrum to derive composite transfer functions of saidplurality of audio transfer systems.

[0055] A multi-channel echo cancel method of this invention is a methodwherein, with respect to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones and forming aplurality of audio transfer systems in which multi-channel soundsreproduced by said respective loudspeakers and having a correlation witheach other are collected by said microphones, composite transferfunctions of said plurality of audio transfer systems are estimated soas to set corresponding filter characteristics, respectively, echocancel signals are respectively produced by giving said set filtercharacteristics to composite signals of individual signals to bereproduced by said respective loudspeakers, and said echo cancel signalsare subtracted from composite signals of individual collected audiosignals of said one or plurality of microphones, thereby performing echocancellation, and wherein, using as reference signals a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals (e.g. suitably combining saidmulti-channel audio signals to produce a plurality of low-correlationcomposite signals having a lower correlation with each other than thatbetween said multi-channel audio signals and using a set of saidplurality of low-correlation composite signals as reference signals, ordirectly inputting a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combining saidmulti-channel audio signals and which have a lower correlation with eachother than that between said multi-channel audio signals and using theset of said plurality of low-correlation composite signals as referencesignals, or the like), estimated errors of composite transfer functionsof said plurality of audio transfer systems are respectively derived,thereby to update corresponding filter characteristics to values thatcancel said estimated errors. According to this invention, using asreference signals a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combiningmulti-channel audio signals having a correlation therebetween and whichhave a lower correlation with each other than that between suchmulti-channel audio signals, the estimated errors of the compositetransfer functions of said plurality of audio transfer systems arerespectively derived so as to successively update the correspondingfilter characteristics to the values that cancel said estimated errors,thereby to enable echo cancellation. In accordance therewith, since themulti-channel audio signals can be reproduced from the loudspeakers withno or less processing, which induces deterioration, applied to themulti-channel audio signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. It is also possible to update thefilter characteristics in real time. The calculation of respectivelyderiving the estimated errors of the composite transfer functions ofsaid plurality of audio transfer systems using the set of the pluralityof low-correlation composite signals as the reference signals, may be,for example, a calculation of respectively deriving the estimated errorsof the composite transfer functions of said plurality of audio transfersystems based on a cross-spectrum calculation between said plurality oflow-correlation composite signals and echo cancel error signals obtainedby subtracting the corresponding echo cancel signals from the compositesignals of the individual collected audio signals of said one orplurality of microphones. Further, the calculation of respectivelyderiving the estimated errors of the composite transfer functions ofsaid plurality of audio transfer systems based on said cross-spectrumcalculation, may be, for example, a calculation of combining saidmulti-channel audio signals through addition or subtraction to produce aplurality of low-correlation composite signals having a lowercorrelation with each other than that between said multi-channel audiosignals, deriving cross spectra between said plurality oflow-correlation composite signals and the echo cancel error signalsobtained by subtracting the corresponding echo cancel signals from thecomposite signals of the individual collected audio signals of said oneor plurality of microphones, and ensemble-averaging them in apredetermined time period per cross spectrum to derive estimated errorsof the composite transfer functions of said plurality of audio transfersystems. Further, the correlation between said plurality oflow-correlation composite signals is detected and, when a value of saidcorrelation is no less than a prescribed value, updating of said filtercharacteristics is stopped, thereby to prevent the echo cancel errorsignals from unexpectedly increasing.

[0056] A multi-channel sound transfer method of this invention is suchthat, with respect to two spaces each forming said plurality of audiotransfer systems, any of the foregoing multi-channel echo cancel methodsis carried out, so that the multi-channel audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the multi-channel audiotransmission with reduced echo cancellation can be performed between twospots, which, for example, can be applied to the teleconferencing systemor the like.

[0057] A stereo echo cancel method of this invention is a methodwherein, with respect to a space provided therein with two loudspeakersand one or two microphones and forming two or four audio transfersystems in which stereo sounds reproduced by said respectiveloudspeakers are collected by said microphones, composite transferfunctions of said two or four audio transfer systems are estimated so asto set corresponding filter characteristics, respectively, echo cancelsignals are respectively produced by giving said set filtercharacteristics to composite signals of individual signals to bereproduced by said respective loudspeakers, and said echo cancel signalsare subtracted from composite signals of individual collected audiosignals of said one or two microphones, thereby performing echocancellation, and wherein, using a sum signal and a difference signal ofsaid stereo audio signals as reference signals, composite transferfunctions of said two or four audio transfer systems are respectivelyderived, thereby to set corresponding filter characteristics. Accordingto this invention, since the sum signal and the difference signal of thestereo audio signals have a low correlation therebetween, estimatederrors of the composite transfer functions of the two or four audiotransfer systems are respectively derived using the sum signal and thedifference signal as reference signals, so as to successively update thecorresponding filter characteristics to values that cancel saidestimated errors, thereby to enable echo cancellation. In accordancetherewith, since the stereo signals can be reproduced from theloudspeakers with no or less processing, which induces deterioration,applied to the stereo signals, excellent reproduced tone quality can beachieved. Further, there is no or only a small delay in reproducedsignals. Thus, when applying to the teleconferencing system or the like,natural conversation can be conducted. The calculation of respectivelyderiving the composite transfer functions of said two or four audiotransfer systems using the sum signal and the difference signal of saidstereo audio signals as the reference signals, may be, for example, acalculation of respectively deriving the composite transfer functions ofsaid two or four audio transfer systems based on a cross-spectrumcalculation between the sum signal and the difference signal, and thecomposite signals of the individual collected audio signals of therespective microphones. Further, the calculation of respectivelyderiving the composite transfer functions of said two or four audiotransfer systems based on said cross-spectrum calculation, may be, forexample, a calculation of deriving cross spectra between the sum signaland the difference signal of said stereo audio signals and the compositesignals of the individual collected audio signals of the respectivemicrophones, and ensemble-averaging them in a predetermined time periodper cross spectrum to derive composite transfer functions of said two orfour audio transfer systems.

[0058] A stereo echo cancel method of this invention is a methodwherein, with respect to a space provided therein with two loudspeakersand one or two microphones and forming two or four audio transfersystems in which stereo sounds reproduced by said respectiveloudspeakers are collected by said microphones, composite transferfunctions of said two or four audio transfer systems are estimated so asto set corresponding filter characteristics, respectively, echo cancelsignals are respectively produced by giving said set filtercharacteristics to composite signals of individual signals to bereproduced by said respective loudspeakers, and said echo cancel signalsare subtracted from composite signals of individual collected audiosignals of said one or two microphones, thereby performing echocancellation, and wherein, using a sum signal and a difference signal ofsaid stereo audio signals as reference signals, estimated errors ofcomposite transfer functions of said two or four audio transfer systemsare respectively derived, thereby to update corresponding filtercharacteristics to values that cancel said estimated errors. Accordingto this invention, using as reference signals a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining multi-channel audio signals having a correlationtherebetween and which have a lower correlation with each other thanthat between such multi-channel audio signals, the estimated errors ofthe composite transfer functions of said plurality of audio transfersystems are respectively derived, so as to successively update thecorresponding filter characteristics to the values that cancel theestimated errors, thereby to enable echo cancellation. In accordancetherewith, since the multi-channel audio signals can be reproduced fromthe loudspeakers with no or less processing, which inducesdeterioration, applied to the multi-channel audio signals, excellentreproduced tone quality can be achieved. Further, there is no or only asmall delay in reproduced signals. Thus, when applying to theteleconferencing system or the like, natural conversation can beconducted. It is also possible to update the filter characteristics inreal time. The calculation of respectively deriving the estimated errorsof the composite transfer functions of said two or four audio transfersystems using the sum signal and the difference signal of said stereoaudio signals as the reference signals, may be, for example, acalculation of respectively deriving the estimated errors of thecomposite transfer functions of said two or four audio transfer systemsbased on a cross-spectrum calculation between the sum signal and thedifference signal of said stereo audio signals and respective echocancel error signals obtained by subtracting the corresponding echocancel signals from the composite signals of the individual collectedaudio signals of said one or two microphones. Further, the calculationof respectively deriving the estimated errors of the composite transferfunctions of said two or four audio transfer systems based on thecross-spectrum calculation between the sum signal and the differencesignal of said stereo audio signals and said echo cancel error signals,may be, for example, a calculation of deriving cross spectra between thesum signal and the difference signal of said stereo audio signals andsaid echo cancel error signals, and ensemble-averaging them in apredetermined time period per cross spectrum to derive estimated errorsof the composite transfer functions of said two or four audio transfersystems. Further, the correlation between the sum signal and thedifference signal of said stereo audio signals is detected and, when avalue of said correlation is no less than a prescribed value, updatingof said filter characteristics is stopped, thereby to prevent the echocancel error signals from unexpectedly increasing.

[0059] A stereo audio transmission method of this invention is suchthat, with respect to two spaces each forming said four audio transfersystems, any of the foregoing multi-channel echo cancel methods iscarried out, so that the stereo audio signals, which have beenecho-canceled by performing said method, are transmitted between saidtwo spaces. In accordance therewith, the stereo audio transmission withreduced echo cancellation can be performed between two spots, which, forexample, can be applied to the teleconferencing system or the like.

[0060] A stereo echo canceller of this invention is a stereo echocanceller wherein, with respect to a space provided therein with twoloudspeakers and two microphones and forming four audio transfer systemsin which stereo sounds reproduced by said respective loudspeakers arecollected by said respective microphones, a sum signal of stereo audiosignals to be reproduced by said respective loudspeakers is subjected toconvolution calculations by first and second filter means, respectively,so as to produce first and second echo cancel signals, a differencesignal of the stereo audio signals to be reproduced by said respectiveloudspeakers is subjected to convolution calculations by third andfourth filter means, respectively, so as to produce third and fourthecho cancel signals, echo cancellation is performed by subtracting,using first subtracting means, said first and third echo cancel signalsfrom a sum signal of collected audio signals of the respectivemicrophones, and echo cancellation is performed by subtracting, usingsecond subtracting means, said second and fourth echo cancel signalsfrom a difference signal of the collected audio signals of therespective microphones, said stereo echo canceller comprising: transferfunction calculating means for respectively deriving filtercharacteristics corresponding to composite transfer functions of saidfour audio transfer systems based on a cross-spectrum calculationbetween the sum signal and the difference signal of the stereo audiosignals to be reproduced by said respective loudspeakers and the sumsignal and the difference signal of the respective microphone collectedaudio signals, thereby to set said derived filter characteristics tocorresponding ones of said first to fourth filter means, respectively.

[0061] A stereo echo canceller of this invention is a stereo echocanceller wherein, with respect to a space provided therein with twoloudspeakers and two microphones and forming four audio transfer systemsin which stereo sounds reproduced by said respective loudspeakers arecollected by said respective microphones, a sum signal of stereo audiosignals to be reproduced by said respective loudspeakers is subjected toconvolution calculations by first and second filter means, respectively,so as to produce first and second echo cancel signals, a differencesignal of the stereo audio signals to be reproduced by said respectiveloudspeakers is subjected to convolution calculations by third andfourth filter means, respectively, so as to produce third and fourthecho cancel signals, echo cancellation is performed by subtracting,using first subtracting means, said first and third echo cancel signalsfrom a sum signal of collected audio signals of the respectivemicrophones, and echo cancellation is performed by subtracting, usingsecond subtracting means, said second and fourth echo cancel signalsfrom a difference signal of the collected audio signals of therespective microphones, said stereo echo canceller comprising: transferfunction calculating means for respectively deriving estimated errors ofcomposite transfer functions of said four audio transfer systems basedon a cross-spectrum calculation between the sum signal and thedifference signal of the stereo audio signals to be reproduced by saidrespective loudspeakers and respective echo cancel error signalsobtained by subtracting the corresponding echo cancel signals from thesum signal and the difference signal of the respective microphonecollected audio signals, thereby to update filter characteristics ofsaid first to fourth filter means to values that cancel said estimatederrors, respectively.

[0062] The stereo echo canceller of this invention may be furtherprovided with correlation detecting means for detecting the correlationbetween the sum signal and the difference signal of said stereo audiosignals and, when a value of said correlation is no less than aprescribed value, stopping updating of said filter characteristics,thereby to prevent the echo cancel error signals from unexpectedlyincreasing.

[0063] A stereo sound transfer apparatus of this invention is such that,with respect to two spaces each forming said four audio transfersystems, any of said stereo echo cancellers is arranged in each space,so that the stereo audio signals, which have been echo-canceled by saidstereo echo cancellers, are transmitted between said two spaces.

[0064] [Inventions of Claims 36 to 40 and Inventions Relating to SuchInventions]

[0065] A transfer function calculation apparatus of this invention is atransfer function calculation apparatus which, with respect to a spaceprovided therein with a plurality of loudspeakers and one or a pluralityof microphones and forming a plurality of audio transfer systems inwhich multi-channel sounds inputted from an outside and reproduced bysaid respective loudspeakers and having a correlation with each otherare collected by said microphones, estimates individual transferfunctions of said plurality of audio transfer systems or a plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions, wherein, using as reference signals a setof a plurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that betweensaid multi-channel audio signals (e.g. suitably combining saidmulti-channel audio signals to produce a plurality of low-correlationcomposite signals having a lower correlation with each other than thatbetween said multi-channel audio signals and using a set of saidplurality of low-correlation composite signals as reference signals, ordirectly inputting a set of a plurality of low-correlation compositesignals which correspond to signals obtained by suitably combining saidmulti-channel audio signals and which have a lower correlation with eachother than that between said multi-channel audio signals and using theset of said plurality of low-correlation composite signals as referencesignals, or the like), individual transfer functions of the respectiveaudio transfer systems or a plurality of composite transfer functionsobtained by suitably combining said individual transfer functions areestimated. The calculation of respectively deriving the individualtransfer functions of the respective audio transfer systems or theplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions, using as the reference signals theset of the plurality of low-correlation composite signals, may be, forexample, a calculation of respectively deriving the individual transferfunctions of the respective audio transfer systems or the plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions, based on a cross-spectrum calculationbetween the plurality of low-correlation composite signals and theindividual collected audio signals of the microphones, or the pluralityof composite signals obtained by suitably combining said individualcollected audio signals. Further, the calculation of respectivelyderiving the individual transfer functions of said plurality of audiotransfer systems or the plurality of composite transfer functionsobtained by suitably combining said individual transfer functions, basedon said cross-spectrum calculation, may be, for example, a calculationof respectively deriving the individual transfer functions of saidplurality of audio transfer systems or the plurality of compositetransfer functions obtained by suitably combining said individualtransfer functions, by combining said multi-channel audio signalsthrough addition or subtraction to produce a plurality oflow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals, deriving crossspectra between said plurality of low-correlation composite signals andthe individual collected audio signals of the microphones, or theplurality of composite signals obtained by suitably combining saidindividual collected audio signals, and ensemble-averaging them in apredetermined time period per cross spectrum. The calculation ofrespectively deriving the individual transfer functions of saidplurality of audio transfer systems based on said cross-spectrumcalculation may also be a calculation of respectively deriving theindividual transfer functions of said plurality of audio transfersystems by producing a plurality of mutually orthogonal uncorrelatedcomposite signals by applying a principal component analysis to saidmulti-channel audio signals, deriving cross spectra between saidplurality of uncorrelated composite signals and the individual collectedaudio signals of the microphones, and ensemble-averaging them in apredetermined time period per cross spectrum.

[0066] A transfer function calculation apparatus of this invention is atransfer function calculation apparatus which, with respect to a spaceprovided therein with two loudspeakers and two microphones and formingfour audio transfer systems in which stereo sounds reproduced by saidrespective loudspeakers are collected by said respective microphones,estimates individual transfer functions of said four audio transfersystems or a plurality of composite transfer functions obtained bysuitably combining said individual transfer functions, wherein, using asum signal and a difference signal of said stereo audio signals asreference signals, individual transfer functions of said four audiotransfer systems or a plurality of composite transfer functions obtainedby suitably combining said individual transfer functions are estimated.The calculation of respectively deriving the individual transferfunctions of said four audio transfer systems or the plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions, using the sum signal and the differencesignal of said stereo audio signals as the reference signals, may be,for example, a calculation of respectively deriving the individualtransfer functions of said four audio transfer systems or the pluralityof composite transfer functions obtained by suitably combining saidindividual transfer functions, based on a cross-spectrum calculationbetween said sum signal and said difference signal, and individualcollected audio signals of the microphones, or a plurality of compositesignals obtained by suitably combining said individual collected audiosignals. Further, the calculation of respectively deriving theindividual transfer functions of said four audio transfer systems or theplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions, based on said cross-spectrumcalculation, may be a calculation of deriving cross spectra between thesum signal and the difference signal of said stereo audio signals andthe individual collected audio signals of the microphones, or theplurality of composite signals obtained by suitably combining saidindividual collected audio signals, and ensemble-averaging them in apredetermined time period per cross spectrum, thereby to respectivelyderive the individual transfer functions of said four audio transfersystems or the plurality of composite transfer functions obtained bysuitably combining said individual transfer functions.

[0067] A transfer function calculation apparatus of this invention is atransfer function calculation apparatus which, with respect to a spaceprovided therein with two loudspeakers and two microphones and formingfour audio transfer systems in which stereo sounds reproduced by saidrespective loudspeakers are collected by said respective microphones,estimates individual transfer functions of said four audio transfersystems, wherein mutually orthogonal two uncorrelated composite signalsare produced by applying a principal component analysis to said stereoaudio signals, and individual transfer functions of said four audiotransfer systems are estimated using a set of said two uncorrelatedcomposite signals as reference signals. The calculation of respectivelyderiving the individual transfer functions of said four audio transfersystems using said two uncorrelated composite signals as the referencesignals, may be, for example, a calculation of respectively deriving theindividual transfer functions of said four audio transfer systems basedon a cross-spectrum calculation between said two uncorrelated compositesignals and the individual collected audio signals of the microphones.Further, the calculation of respectively deriving the individualtransfer functions of said four audio transfer systems based on saidcross-spectrum calculation, may be, for example, a calculation ofderiving cross spectra between said two uncorrelated composite signalsand the individual collected audio signals of the microphones, andensemble-averaging them in a predetermined time period per crossspectrum, thereby to respectively derive the individual transferfunctions of said four audio transfer systems. In this case, by makingrelatively longer a time period of performing said ensemble averagingwhen double talk is detected where sounds other than those reproduced bysaid loudspeakers are inputted into said microphones, while making itrelatively shorter when the double talk is not detected, it is possibleto fully converge the estimated errors when the double talk exists, andfurther, quicken the convergence of the estimated errors when there isno double talk.

[0068] [Inventions of Claims 41 to 51 and Inventions Relating to SuchInventions]

[0069] An inventive echo cancel method is associated to a space providedtherein with a plurality of loudspeakers and one or a plurality ofmicrophones for forming a plurality of audio transfer systems throughwhich audio signals of multi-channels having a correlation with eachother are reproduced by said respective loudspeakers and are collectedby said microphones, and designed for performing an echo cancellation bysubtracting an echo cancel signal from the audio signals collected bythe respective microphone or from composite signals obtained bycombining the collected audio signals. The inventive method comprisesinputting a plurality of low-correlation audio signals which areobtained by suitably combining first audio signals of multi-channels andwhich have a lower correlation with each other than that among saidfirst audio signals of multi-channels, generating second audio signalsof multi-channels having a correlation with each other by computationbased on the inputted low-correlation audio signals, feeding thegenerated second audio signals to the respective loudspeakers so as toreproduce audio sounds, feeding the generated second audio signals orthe inputted low-correlation audio signals to filters, estimatingindividual transfer functions of said plurality of said audio transfersystems or a plurality of composite transfer functions obtained bysuitably combining said individual transfer functions based on theinputted low-correlation audio signals so as to set corresponding filtercharacteristics, producing echo cancel signals by applying said setfilter characteristics to the second audio signals or thelow-correlation audio signals fed to the filters, and subtracting saidecho cancel signals from collected audio signals obtained by collectingthe reproduced audio sounds by the microphones or from composite audiosignals obtained by suitably combining said collected audio signals,thereby performing the echo cancellation.

[0070] Preferably, in the inventive echo cancel method, the inputtedlow-correlation audio signals are obtained by adding or subtracting thefirst audio signals of multi-channels with each other.

[0071] Another inventive echo cancel method is associated to a spaceprovided therein with a plurality of loudspeakers and one or a pluralityof microphones for forming a plurality of audio transfer systems throughwhich audio signals of multi-channels having a correlation with eachother are reproduced by said respective loudspeakers and are collectedby said microphones, and designed for performing an echo cancellation bysubtracting an echo cancel signal from the audio signals collected bythe respective microphone or from composite signals obtained bycombining the collected audio signals. The inventive method comprisesinputting a plurality of first low-correlation audio signals which areobtained by suitably combining first audio signals of multi-channels andwhich have a lower correlation with each other than that among saidfirst audio signals of multi-channels, generating second audio signalsof multi-channels having a correlation with each other by computationbased on the inputted first low-correlation audio signals, feeding thegenerated second audio signals to the respective loudspeakers so as toreproduce audio sounds, generating second low-correlation audio signalsof multi-channels based on the generated second audio signals, feedingthe generated second audio signals or the generated secondlow-correlation audio signals to filters, estimating individual transferfunctions of said plurality of said audio transfer systems or aplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions based on the generated secondlow-correlation audio signals so as to set corresponding filtercharacteristics, producing echo cancel signals by applying said setfilter characteristics to the second audio signals or the secondlow-correlation audio signals fed to the filters, and subtracting saidecho cancel signals from collected audio signals obtained by collectingthe reproduced audio sounds at the microphones or from composite audiosignals obtained by suitably combining said collected audio signals,thereby performing the echo cancellation.

[0072] Preferably, in the inventive echo cancel method, the inputtedfirst low-correlation audio signals are obtained by adding orsubtracting the first audio signals of multi-channels with each other.

[0073] An inventive echo canceller is associated to a space providedtherein with a plurality of loudspeakers and one or a plurality ofmicrophones for forming a plurality of audio transfer systems throughwhich audio signals of multi-channels having a correlation with eachother are reproduced by said respective loudspeakers and are collectedby said microphones, and designed for performing an echo cancellation bysubtracting an echo cancel signal from the audio signals collected bythe respective microphone or from composite signals obtained bycombining the collected audio signals. The inventive echo cancellercomprises an inputting means for inputting a plurality oflow-correlation audio signals which are obtained by suitably combiningfirst audio signals of multi-channels and which have a lower correlationwith each other than that among said first audio signals ofmulti-channels, a demodulating means provided for generating secondaudio signals of multi-channels having a correlation with each other bydemodulating the inputted low-correlation audio signals, and for feedingthe generated second audio signals to the respective loudspeakers so asto reproduce audio sounds, an estimating means for estimating individualtransfer functions of said plurality of said audio transfer systems or aplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions based on the inputted low-correlationaudio signals so as to set corresponding filter characteristics, afilter means for producing echo cancel signals by applying said setfilter characteristics to the second audio signals or thelow-correlation audio signals fed to the filter means, and a subtractingmeans for subtracting said echo cancel signals from collected audiosignals obtained by collecting the reproduced audio sounds at themicrophones or from composite audio signals obtained by suitablycombining said collected audio signals, thereby performing the echocancellation.

[0074] Preferably, in the inventive echo canceller, the inputtedlow-correlation audio signals are obtained by adding or subtracting thefirst audio signals of multi-channels with each other.

[0075] Another inventive echo canceller is associated to a spaceprovided therein with a plurality of loudspeakers and one or a pluralityof microphones for forming a plurality of audio transfer systems throughwhich audio signals of multi-channels having a correlation with eachother are reproduced by said respective loudspeakers and are collectedby said microphones, and designed for performing an echo cancellation bysubtracting an echo cancel signal from the audio signals collected bythe respective microphone or from composite signals obtained bycombining the collected audio signals. The inventive echo cancellercomprises an inputting means for inputting a plurality of firstlow-correlation audio signals which are obtained by suitably combiningfirst audio signals of multi-channels and which have a lower correlationwith each other than that among said first audio signals ofmulti-channels, a demodulating means provided for generating secondaudio signals of multi-channels having a correlation with each other bydemodulating the inputted first low-correlation audio signals, and forfeeding the generated second audio signals to the respectiveloudspeakers so as to reproduce audio sounds, an estimating meansprovided for generating second low-correlation audio signals ofmulti-channels based on the generated second audio signals, and forestimating individual transfer functions of said plurality of said audiotransfer systems or a plurality of composite transfer functions obtainedby suitably combining said individual transfer functions based on thegenerated second low-correlation audio signals so as to setcorresponding filter characteristics, a filter means for producing echocancel signals by applying said set filter characteristics to the secondaudio signals or the second low-correlation audio signals fed to thefilter means, and a subtracting means for subtracting said echo cancelsignals from collected audio signals obtained by collecting thereproduced audio sounds at the microphones or from composite audiosignals obtained by suitably combining said collected audio signals,thereby performing the echo cancellation.

[0076] Preferably, in the inventive echo canceller, the inputted firstlow-correlation audio signals are obtained by adding or subtracting thefirst audio signals of multi-channels with each other.

[0077] Preferably, in a inventive multi-channel echo canceller, themulti-channel audio signals being inputted from an outside and having acorrelation with each other are reproduced by said respectiveloudspeakers without lowering the correlation of the inputtedmulti-channel audio signals.

[0078] Preferably, the multi-channel audio signals being inputted froman outside and having a correlation with each other are provisionallymodulated to lower the correlation, then demodulated to restore thecorrelation, and thereafter reproduced by said respective loudspeakers.Further, the multi-channel audio signals are provisionally modulated tolower the correlation by adding and subtracting the multi-channel audiosignals with each other, or by orthogonalizing the multi-channel audiosignals with each other.

[0079] In this invention, “audio signal” is not limited to human voices,but covers all acoustic signals in the audible frequency band.

BRIEF DESCRIPTION OF THE DRAWINGS

[0080]FIG. 1 is a block diagram showing a structural example in a stereoecho canceller 16, 24 of FIG. 2.

[0081]FIG. 2 is a block diagram showing an embodiment of a stereo soundtransfer apparatus of this invention.

[0082]FIG. 3 is a diagram showing the simulation measurement results ofthe echo cancel performance of the stereo echo canceller 16, 24 of FIG.1.

[0083]FIG. 4 is a diagram showing the simulation measurement results ofthe echo cancel performance of the stereo echo canceller 16, 24 of FIG.1.

[0084]FIG. 5 is a diagram showing the simulation measurement results ofthe echo cancel performance of the stereo echo canceller 16, 24 of FIG.1.

[0085]FIG. 6 is a diagram showing the simulation measurement results ofthe echo cancel performance of the stereo echo canceller 16, 24 of FIG.1.

[0086]FIG. 7 is a diagram showing the simulation measurement results ofthe echo cancel performance of the stereo echo canceller 16, 24 of FIG.1.

[0087]FIG. 8 is a diagram showing the simulation measurement results ofthe echo cancel performance of the stereo echo canceller 16, 24 of FIG.1.

[0088]FIG. 9 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0089]FIG. 10 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0090]FIG. 11 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0091]FIG. 12 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0092]FIG. 13 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0093]FIG. 14 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0094]FIG. 15 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0095]FIG. 16 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0096]FIG. 17 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0097]FIG. 18 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0098]FIG. 19 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0099]FIG. 20 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0100]FIG. 21 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0101]FIG. 22 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0102]FIG. 23 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0103]FIG. 24 is a block diagram showing a modification of thestructures of FIGS. 1, 9 to 11.

[0104]FIG. 25 is a block diagram showing a modification of thestructures of FIGS. 12 to 15.

[0105]FIG. 26 is a block diagram showing a modification of thestructures of FIGS. 16 to 19.

[0106]FIG. 27 is a block diagram showing a modification of thestructures of FIGS. 20 and 21.

[0107]FIG. 28 is a block diagram showing another structural example inthe stereo echo canceller 16, 24 of FIG. 2.

[0108]FIG. 29 is a time chart showing an example of unit intervals fororthogonalization processing and deriving an impulse response or itsestimated error in the stereo echo canceller of FIG. 28.

[0109]FIG. 30 is a diagram showing one example of functional blocks ofan orthogonalizing filter 500 of FIG. 28.

[0110]FIG. 31 is a diagram showing one example of functional blocks oftransfer function calculating means 502 of FIG. 28.

[0111]FIG. 32 is a functional block diagram showing a modification ofthe transfer function calculating means 502 of FIG. 31.

[0112]FIG. 33 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in echo cancellation amount ofthe stereo echo canceller 16, 24 of FIG. 28 when there is no doubletalk.

[0113]FIG. 34 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in echo cancellation amount ofthe stereo echo canceller 16, 24 of FIG. 28 when there is no doubletalk.

[0114]FIG. 35 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in echo cancellation amount ofthe stereo echo canceller 16, 24 of FIG. 28 when there is no doubletalk.

[0115]FIG. 36 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in echo cancellation amount ofthe stereo echo canceller 16, 24 of FIG. 28 when there is no doubletalk.

[0116]FIG. 37 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in echo cancellation amount ofthe stereo echo canceller 16, 24 of FIG. 28 when there is double talk.

[0117]FIG. 38 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in echo cancellation amount ofthe stereo echo canceller 16, 24 of FIG. 28 when there is double talk.

[0118]FIG. 39 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in echo cancellation amount ofthe stereo echo canceller 16, 24 of FIG. 28 when there is double talk.

[0119]FIG. 40 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in echo cancellation amount ofthe stereo echo canceller 16, 24 of FIG. 28 when there is double talk.

[0120]FIG. 41 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in transfer function estimatederror of the stereo echo canceller 16, 24 of FIG. 28 when there is nodouble talk.

[0121]FIG. 42 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in transfer function estimatederror of the stereo echo canceller 16, 24 of FIG. 28 when there is nodouble talk.

[0122]FIG. 43 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in transfer function estimatederror of the stereo echo canceller 16, 24 of FIG. 28 when there is nodouble talk.

[0123]FIG. 44 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in transfer function estimatederror of the stereo echo canceller 16, 24 of FIG. 28 when there is nodouble talk.

[0124]FIG. 45 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in transfer function estimatederror of the stereo echo canceller 16, 24 of FIG. 28 when there isdouble talk.

[0125]FIG. 46 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in transfer function estimatederror of the stereo echo canceller 16, 24 of FIG. 28 when there isdouble talk.

[0126]FIG. 47 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in transfer function estimatederror of the stereo echo canceller 16, 24 of FIG. 28 when there isdouble talk.

[0127]FIG. 48 is a diagram showing the simulation measurement resultwith respect to a time-domain variation in transfer function estimatederror of the stereo echo canceller 16, 24 of FIG. 28 when there isdouble talk.

[0128]FIG. 49 is an exemplary diagram for explaining an ensembleaveraging process according to overlap processing.

[0129]FIG. 50 is a block diagram showing a modification of the stereoecho canceller 16, 24 of FIG. 28.

[0130]FIG. 51 is a block diagram showing a structural example whereinthe number of microphones is modified to one in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

[0131] Embodiments of the present invention will be describedhereinbelow. FIG. 2 shows the whole structure of a two-way stereo soundtransfer apparatus according to this invention. This is for performingtwo-way stereo transmission between a spot A and a spot B and, forexample, is applicable to the teleconferencing system. In the spot A,two loudspeakers SP-A(L) and SP-A(R) and two microphones MC-A(L) andMC-A(R) are arranged in one space. Collected audio signals of themicrophones MC-A(L) and MC-A(R) are converted into digital signals atA/D converters 12 and 14, respectively, and applied with echo cancelprocessing at a stereo echo canceller 16, then modulated at a CODEC(CODER and DECODER) 18 and transmitted to the spot B via a wire or radiotransmission line 20. In the spot B, two loudspeakers SP-B(L) andSP-B(R) and two microphones MC-B(L) and MC-B(R) are arranged in onespace. Incidentally, assuming a situation that participants in the spotsA and B talk with each other in a face-to-face manner in theteleconferencing system, loudspeakers and microphones are arranged suchthat a sound collected by a microphone on the participant's left in onespot is reproduced from a loudspeaker on the participant's right in theother spot whereas a sound collected by a microphone on theparticipant's right in one spot is reproduced from a loudspeaker on theparticipant's left in the other spot. Specifically, when the loudspeakerSP-A(L) and the microphone MC-A(L) are arranged on the participant'sleft and the loudspeaker SP-A(R) and the microphone MC-A(R) are arrangedon the participant's right in the spot A, the loudspeaker SP-B(L) andthe microphone MC-B(L) are arranged on the participant's right and theloudspeaker SP-B(R) and the microphone MC-B(R) are arranged on theparticipant's left in the spot B.

[0132] The signals transmitted from the spot A are inputted into a CODEC22 where the collected audio signals of the microphones MC-A(L) andMC-A(R) are demodulated. These demodulated collected audio signals ofthe microphones MC-A(L) and MC-A(R) are respectively converted intoanalog signals at D/A converters 26 and 28 via a stereo echo canceller24, and respectively reproduced at the loudspeakers SP-B(L) and SP-B(R).Collected audio signals of the microphones MC-B(L) and MC-B(R) in thespot B are converted into digital signals at A/D converters 30 and 32,respectively, and applied with echo cancel processing at the stereo echocanceller 24, then modulated at the CODEC 22 and transmitted to the spotA via the transmission line 20. The signals transmitted to the spot Aare inputted into the CODEC 18 where the collected audio signals of themicrophones MC-B(L) and MC-B(R) are demodulated. These demodulatedcollected audio signals of the microphones MC-B(L) and MC-B(R) arerespectively converted into analog signals at D/A converters 34 and 36via the stereo echo canceller 16, and respectively reproduced at theloudspeakers SP-A(L) and SP-A(R).

[0133]FIG. 1 shows a structural example in the stereo echo canceller 16,24. Left/right two-channel stereo signals x_(L) and x_(R) transmittedfrom the spot on the counterpart side and inputted into line input endsLI(L) and LI (R) are outputted from sound output ends SO(L) and SO(R) asthey are (i.e. not through sum/difference signal producing means 52),and reproduced at loudspeakers SP(L) {representing SP-A(L) or SP-B(L)}and SP(R) {representing SP-A(R) or SP-B(R)}, respectively.

[0134] Filer means 40-1 to 40-4 are formed by, for example, FIR filters.Of them, the filter means 40-1 is set with an impulse responsecorresponding to a transfer function between the loudspeaker SP(L) and amicrophone MC(L) {representing MC-A(L) or MC-B(L)} and performs, usingsuch an impulse response, a convolution calculation of a signal to beoutputted from the sound output end SO(L), thereby producing an echocancel signal EC1 corresponding to a signal obtained such that thesignal outputted from the sound output end SO(L) is reproduced at theloudspeaker SP(L), collected by the microphone MC(L) and inputted into asound input end SI(L). The filter means 40-2 is set with an impulseresponse corresponding to a transfer function between the loudspeakerSP(L) and a microphone MC(R) {representing MC-A(R) or MC-B(R)} andperforms, using such an impulse response, a convolution calculation of asignal to be outputted from the sound output end SO(L), therebyproducing an echo cancel signal EC2 corresponding to a signal obtainedsuch that the signal outputted from the sound output end SO(L) isreproduced at the loudspeaker SP(L), collected by the microphone MC(R)and inputted into a sound input end SI(R). The filter means 40-3 is setwith an impulse response corresponding to a transfer function betweenthe loudspeaker SP(R) and the microphone MC(L) and performs, using suchan impulse response, a convolution calculation of a signal to beoutputted from the sound output end SO(R), thereby producing an echocancel signal EC3 corresponding to a signal obtained such that thesignal outputted from the sound output end SO(R) is reproduced at theloudspeaker SP(R), collected by the microphone MC(L) and inputted intothe sound input end SI(L). The filter means 40-4 is set with an impulseresponse corresponding to a transfer function between the loudspeakerSP(R) and the microphone MC(R) and performs, using such an impulseresponse, a convolution calculation of a signal to be outputted from thesound output end SO(R), thereby producing an echo cancel signal EC4corresponding to a signal obtained such that the signal outputted fromthe sound output end SO(R) is reproduced at the loudspeaker SP(R),collected by the microphone MC(R) and inputted into the sound input endSI (R).

[0135] An adder 44 performs a calculation of EC1+EC3. An adder 46performs a calculation of EC2+EC4. A subtracter 48 subtracts an echocancel signal EC1+EC3 from a collected audio signal of the microphoneMC(L) inputted from the sound input end SI(L), thereby to perform echocancellation. A subtracter 50 subtracts an echo cancel signal EC2+EC4from a collected audio signal of the microphone MC(R) inputted from thesound input end SI(R), thereby to perform echo cancellation. Theseecho-canceled signals of the respective left and right channels areoutputted from line output ends LO(L) and LO(R), respectively, andtransmitted toward the spot on the counterpart side.

[0136] The sum/difference signal producing means 52 performs addition,using an adder 54, of the left/right two-channel stereo signals XL andXR inputted into the line input ends LI (L) and LI (R) so as to producea sum signal x_(L)+x_(R), while performs subtraction thereof using asubtracter 56 so as to produce a difference signal x_(L)−x_(R) (or itmay also be x_(R)−x_(L)). In case of the left/right two-channel stereosignals, the sum signal x_(L)+x_(R) and the difference signalx_(L)−x_(R) are in general low in correlation therebetween, andfrequently, approximately uncorrelated. Transfer function calculatingmeans 58 implements a cross-spectrum calculation between the sum signalx_(L)+x_(R) and the difference signal x_(L)−x_(R) produced by thesum/difference signal producing means 52 and signals e_(L) and e_(R)outputted from the subtracters 48 and 50 and, based on thiscross-spectrum calculation, sets filter characteristics (impulseresponses) of the filter means 40-1 to 40-4. Specifically, upon startingthe system, the filter characteristics of the filter means 40-1 to 40-4are not set, i.e. coefficients are all set to zero, so that the echocancel signals EC1 to EC4 are zero, and thus the collected audio signalsof the microphones MC(L) and MC(R) themselves are outputted from thesubtracters 48 and 50. Therefore, at this time, the transfer functioncalculating means 58 performs the cross-spectrum calculation between thesum signal x_(L)+x_(R) and the difference signal x_(L)-x_(R) produced bythe sum/difference signal producing means 52 and the collected audiosignals e_(L) and e_(R) of the microphones MC(L) and MC(R) outputtedfrom the subtracters 48 and 50 and, based on this cross-spectrumcalculation, derives transfer functions of four audio transfer systemsbetween the loudspeakers SP(L) and SP(R) and the microphones MC(L) andMC(R), respectively, and implements initial setting of the filtercharacteristics of the filter means 40-1 to 40-4 to values correspondingto such transfer functions. After the initial setting, since the echocancel signals are produced by the filter means 40-1 to 40-4, the echocancel error signals e_(L) and e_(R) corresponding to difference signalsbetween the collected audio signals of the microphones MC(L) and MC(R)and the echo cancel signals EC1 to EC4 are outputted from thesubtracters 48 and 50. Therefore, at this time, the transfer functioncalculating means 58 performs the cross-spectrum calculation between thesum signal x_(L)+x_(R) and the difference signal x_(L)-x_(R) produced bythe sum/difference signal producing means 52 and the echo cancel errorsignals e_(L) and e_(R) outputted from the subtracters 48 and 50 and,based on this cross-spectrum calculation, derives estimated errors ofthe transfer functions of the four audio transfer systems between theloudspeakers SP(L) and SP(R) and the microphones MC(L) and MC(R),respectively, and updates the filter characteristics of the filter means40-1 to 40-4 to values that cancel the estimated errors, respectively.By repeating this updating operation per prescribed time period, theecho cancel error can be converged to a minimum value. Further, even ifthe transfer functions change due to movement of the microphonepositions or the like, the echo cancel error can be converged to aminimum value by sequentially updating the filter characteristics of thefilter means 40-1 to 40-4 depending thereon.

[0137] Correlation detecting means 60 detects a correlation between thesum signal x_(L)+x_(R) and the difference signal x_(L)-x_(R) based on acorrelation value calculation or the like, and stops updating of theforegoing filter characteristics when the correlation value is no lessthan a prescribed value. When the correlation value becomes lower thanthe prescribed value, updating of the foregoing filter characteristicsis restarted. Incidentally, as a concrete technique for deriving thecorrelation between the sum signal x_(L)+x_(R) and the difference signalx_(L)-x_(R), any of the known techniques for detecting a correlation oftwo signals may be used.

[0138] Herein, the filter characteristics (impulse responses) that areset to the filter means 40-1 to 40-4 by the transfer functioncalculating means 58 will be described. In the following description,the transfer functions and the filter characteristics are expressedusing the following symbols.

[0139] H_(xx): a transfer function (frequency-axis expression)

[0140] h_(xx): an impulse response (time-axis expression) correspondingto H_(xx)

[0141] H{circumflex over ( )}_(xx): an estimated transfer function (atransfer function set to a filter)

[0142] h{circumflex over ( )}_(xx): an impulse response corresponding toH{circumflex over ( )}_(xx)

[0143] ΔH_(xx): a transfer function estimated error

[0144] Δh_(xx): an impulse response corresponding to ΔH_(xx)

[0145] (note: Suitable symbols are allocated to xx.)

[0146] The sum signal and the difference signal are respectively definedas follows.

sum signal: x _(M)(=x _(L) +x _(R))

difference signal: x _(S)(=x _(L) −x _(R))

[0147] The transfer functions of the four audio transfer systems betweenthe loudspeakers SP(L) and SP(R) and the microphones MC(L) and MC(R) arerespectively defined as follows.

[0148] H_(LL): a transfer function of the system from the loudspeakerSP(L) to the microphone MC(L)

[0149] H_(LR): a transfer function of the system from the loudspeakerSP(L) to the microphone MC(R)

[0150] H_(RL): a transfer function of the system from the loudspeakerSP(R) to the microphone MC(L)

[0151] H_(RR): a transfer function of the system from the loudspeakerSP(R) to the microphone MC(R)

[0152] The input signals x_(L) and x_(R) at the line input ends LI(L)and LI(R) of the stereo echo canceller 16, 24 are replaced by

x _(L)=(x _(M) +x _(S))/2

x _(R)=(x _(M) −x _(S))/2.

[0153] Then, a calculation shown below is carried out in the transferfunction calculating means 58. The signals e_(L) and e_(R) {thecollected audio signals of the microphones MC(L) and MC(R) as they arebefore the initial setting of the filter means 40-1 to 40-4 whereas theecho cancel error signals after the initial setting} outputted from thesubtracters 48 and 50, assuming that frequency-axis expressions of thesignals x_(M), x_(S), e_(L) and e_(R) are respectively given as X_(M),X_(S), E_(L) and E_(R), become

E _(L)={(X _(M) +X _(S))H _(LL)/2}+{(X _(M) −X _(S))H _(RL)/2}

E _(R)={(X _(M) +X _(S))H _(LR)/2}+{(X _(M) −X _(S))H _(RR)/2}

[0154] hence $\begin{matrix}\begin{matrix}{{2E_{L}} = {{\left( {X_{M} + X_{S}} \right)H_{LL}} + {\left( {X_{M} - X_{S}} \right)H_{RL}}}} \\{= {{X_{M}\left( {H_{LL} + H_{RL}} \right)} + {X_{S}\left( {H_{LL} - H_{RL}} \right)}}}\end{matrix} & (1) \\\begin{matrix}{{2E_{R}} = {{\left( {X_{M} + X_{S}} \right)H_{LR}} + {\left( {X_{M} - X_{S}} \right)H_{RR}}}} \\{= {{X_{M}\left( {H_{LR} + H_{RR}} \right)} + {{X_{S}\left( {H_{LR} - H_{RR}} \right)}.}}}\end{matrix} & (2)\end{matrix}$

[0155] When both sides of the equation (1) are multiplied by complexconjugates X_(M)* and X_(S)* of X_(M) and X_(S) (i.e. deriving crossspectra) and ensemble-averaged,

ΣX _(M)*·2E _(L) =ΣX _(M) *·X _(M)(H _(LL) +H _(RL))+ΣX _(M) *·X _(S)(H_(LL) −H _(RL))   (3)

ΣX _(S)*·2E _(L) =ΣX _(S) *·X _(M)(H _(LL) +H _(RL))+ΣX _(S) *·X _(S)(H_(LL) −H _(RL))   (4)

[0156] are respectively obtained. Similarly, when both sides of theequation (2) are multiplied by complex conjugates X_(M)* and X_(S)* ofX_(M) and X_(S),

ΣX _(M)*·2E _(R) =ΣX _(M) *·X _(M)(H _(LR) +H _(RR))+ΣX _(M) *·X _(S)(H_(LR) −H _(RR))   (5)

ΣX _(S)*·2E _(R) =ΣX _(S) *·X _(M)(H _(LR) +H _(RR))+ΣX _(S) *·X _(S)(H_(LR) −H _(RR))   (6)

[0157] are respectively obtained.

[0158] In the equations (3) to (6), since X_(M) and X_(S) areapproximately uncorrelated with each other, such a term havingX_(M)*·X_(S) or X_(S)*·X_(M) becomes approximately zero whenensemble-averaged. Further,

X _(M) *·X _(M) =|X _(M)|²

X _(S) *·X _(S) =|X _(S)|²

[0159] hence, the equations (3) to (6) respectively become

ΣX _(M)*·2E _(L) =Σ|X _(M)|²(H _(LL) +H _(RL))   (3′)

ΣX _(S)*·2E _(L) =Σ|X _(S)|²(H _(LL) +H _(RL))   (4′)

ΣX _(M)*·2E _(R) =Σ|X _(M)|²(H _(LR) +H _(RR))   (5′)

ΣX _(S)*·2E _(R) =Σ|X _(S)|²(H _(LR) +H _(RR))   (6′)

[0160] By transforming the equations (3′) to (6′), the followingcomposite transfer functions each in the form of combination of twotransfer functions are respectively derived.

H _(LL) +H _(RL) =ΣX _(M)*·2E _(L) /ρ|X _(M)|²   (3″)

H _(LL) +H _(RL) =ΣX _(S)*·2E _(L) /ρ|X _(S)|²   (4″)

H _(LR) +H _(RR) =ΣX _(M)*·2E _(R) /ρ|X _(M)|²   (5″)

H _(LR) +H _(RR) =ΣX _(S)*·2E _(R) /ρ|X _(S)|²   (6″)

[0161] When the corresponding sides of the equations (3″) and (4″) areadded together,

H _(LL)=(ΣX _(M) *·E _(L) /Σ|X _(M)|²)+(ΣX _(S) *·E _(L) /Σ|X _(S)|²)  (7).

[0162] When subtraction is performed between the corresponding sides ofthe equations (3″) and (4″),

H _(RL)=(ΣX _(M) *·E _(L) /Σ|X _(M)|²)+(ΣX _(S) *·E _(L) /Σ|X _(S)|²)  (8).

[0163] When the corresponding sides of the equations (5″) and (6″) areadded together,

H _(LR)=(ΣX _(M) *·E _(R) /Σ|X _(M)|²)+(ΣX _(S) *·E _(R) /Σ|X _(S)|²)  (9).

[0164] When subtraction is performed between the corresponding sides ofthe equations (5″) and (6″),

H _(RR)=(ΣX _(M) *·E _(R) /Σ|X _(M)|²)+(ΣX _(S) *·E _(R) /Σ|X _(S)|²)  (10).

[0165] Hence, transfer functions H_(LL), H_(RL), H_(LR) and H_(RR) arerespectively derived. Impulse responses h_(LL), h_(RL), h_(LR) andh_(RR) obtained by applying the inverse Fourier transformation to thesederived transfer functions are the filter characteristics to be set tothe filter means 40-1, 40-2, 40-3 and 40-4, respectively. Therefore, thetransfer function calculating means 58 derives the respective transferfunctions H_(LL), H_(RL), H_(LR) and H_(RR) from the equations (7) to(10) based on the sum signal x_(M), the difference signal x_(S), and theoutput signals e_(L) and e_(R) of the subtracters 48 and 50 that areinputted, derives the impulse responses h_(LL), h_(RL), h_(LR) andh_(RR) by applying the inverse Fourier transformation to those derivedtransfer functions, sets the derived impulse responses to the filtermeans 40-1, 40-2, 40-3 and 40-4 as h{circumflex over ( )}_(LL),h{circumflex over ( )}_(RL), h{circumflex over ( )}_(LR) andH{circumflex over ( )}_(RR), respectively, and further, updates theimpulse responses by repeating this calculation per suitably determinedprescribed time period (e.g. time period of performing ensembleaveraging).

[0166] When the foregoing impulse response updating technique isexplained using estimated error parameters, it becomes as follows.Output signals (collected audio signals) y_(L) and y_(R) of themicrophones MC(L) and MC(R), assuming that frequency-axis expressions ofx_(L), x_(R), y_(L) and y_(R) are respectively given as X_(L), X_(R),Y_(L) and Y_(R), become

Y _(L)=(X _(M) +X _(S))H _(LL)/2+(X _(M) −X _(S))H _(RL)/2

Y _(R)=(X _(M) +X _(S))H _(LR)/2+(X _(M) −X _(S))H _(RR)/2.

[0167] The signal E_(L) outputted from the subtracter 48 becomes$\begin{matrix}{E_{L} = {Y_{L} - \left( {{X_{L} \cdot H_{LL}^{\hat{}}} + {X_{R} \cdot H_{RL}^{\hat{}}}} \right)}} \\{= {\left\{ {{\left( {X_{M} + X_{S}} \right){H_{LL}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RL}/2}}} \right\} -}} \\{\left\{ {{\left( {X_{M} + X_{S}} \right){H_{LL}^{\hat{}}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RL}^{\hat{}}/2}}} \right\}}\end{matrix}$

[0168] hence

2E _(L) =X _(M)(H _(LL) +H _(RL) −H{circumflex over ( )} _(LL)−H{circumflex over ( )} _(RL))+X _(S)(H _(LL) −H _(RL) −H{circumflexover ( )} _(LL) +H{circumflex over ( )} _(RL))   (11).

[0169] The signal E_(R) outputted from the subtracter 50 becomes$\begin{matrix}{E_{R} = {Y_{R} - \left( {{X_{L} \cdot H_{LR}^{\hat{}}} + {X_{R} \cdot H_{RR}^{\hat{}}}} \right)}} \\{= {\left\{ {{\left( {X_{M} + X_{S}} \right){H_{LR}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RR}/2}}} \right\} -}} \\{\left\{ {{\left( {X_{M} + X_{S}} \right){H_{LR}^{\hat{}}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RR}^{\hat{}}/2}}} \right\}}\end{matrix}$

[0170] hence

2E _(R) =X _(M)(H _(LR) +H _(RR) −H{circumflex over ( )} _(LR)−H{circumflex over ( )} _(RR))+X _(S)(H _(LR) −H _(RR) −H{circumflexover ( )} _(LR) +H{circumflex over ( )} _(RR))   (12).

[0171] When the estimated errors are given as

ΔH _(LL) =H _(LL) −H{circumflex over ( )} _(LL)

ΔH _(RL) =H _(RL) −H{circumflex over ( )} _(RL)

ΔH _(LR) =H _(LR) −H{circumflex over ( )} _(LR)

ΔH _(RR) =H _(RR) −H{circumflex over ( )} _(RR)

[0172] the equations (11) and (12) become

2E _(L) =X _(M)(ΔH _(LL) +ΔH _(RL))+X _(S)(ΔH _(LL) −ΔH _(RL))   (11′)

2E _(R) =X _(M)(ΔH _(LR) +ΔH _(RR))+X _(S)(ΔH _(LR) −ΔH _(RR))   (12′).

[0173] When both sides of the equation (11′) are multiplied by complexconjugates X_(M)* and X_(S)* of X_(M) and X_(S) (i.e. deriving crossspectra) and ensemble-averaged,

ΣX _(M)*·2E _(L) =ΣX _(M) *·X _(M)(ΔH _(LL) +ΔH _(RL))+ΣX _(M) *·X_(S)(ΔH _(LL) −ΔH _(RL))   (13)

ΣX _(S)*·2E _(L) =ΣX _(S) *·X _(M)(ΔH _(LL) +ΔH _(RL))+ΣX _(S) *·X_(S)(ΔH _(LL) −ΔH _(RL))   (14)

[0174] are respectively obtained. Similarly, when both sides of theequation (12′) are multiplied by complex conjugates X_(M)* and X_(S)* ofX_(M) and X_(S),

ΣX _(M)*·2E _(R) =ΣX _(M)(ΔH _(LR) +ΔH _(RR))+ΣX _(M) *·X _(S)(ΔH _(LL)−ΔH _(RR))   (15)

ΣX _(S)*·2E _(R) =ΣX _(S)(ΔH _(LR) +ΔH _(RR))+ΣX _(S) *·X _(S)(ΔH _(LL)−ΔH _(RR))   (16)

[0175] are respectively obtained.

[0176] In the equations (13) to (16), since X_(M) and X_(S) areapproximately uncorrelated with each other, such a term havingX_(M)*·X_(S) or X_(S)*·X_(M) becomes approximately zero whenensemble-averaged. Further,

X _(M) *·X _(M) =|X _(M)|²

X _(S) *·X _(S) =|X _(S)|²

[0177] hence, the equations (13) to (16) respectively become

ΣX _(M)*·2E _(L) =Σ|X _(M)|²(ΔH _(LL) +ΔH _(RL))   (13′)

ΣX _(S)*·2E _(L) =Σ|X _(S)|²(ΔH _(LL) +ΔH _(RL))   (14′)

ΣX _(M)*·2E _(R) =Σ|X _(M)|²(ΔH _(LR) +ΔH _(RR))   (15′)

ΣX _(S)*·2E _(R) =Σ|X _(S)|²(ΔH _(LR) +ΔH _(RR))   (16′).

[0178] From the equations (13′) to (16′),

ΔH _(LL) =ΣX _(M) *·E _(L) /Σ|X _(M)|² +ΣX _(S) *·E _(L) /Σ|X _(S)|²  (17)

ΔH _(RL) =ΣX _(M) *·E _(L) /Σ|X _(M)|² +ΣX _(S) *·E _(L) /Σ|X _(S)|²  (18)

ΔH _(LR) =ΣX _(M) *·E _(R) /Σ|X _(M)|² +ΣX _(S) *·E _(R) /Σ|X _(S)|²  (19)

ΔH _(RR) =ΣX _(M) *·E _(R) /Σ|X _(M)|² +ΣX _(S) *·E _(R) /Σ|X _(S)|²  (20)

[0179] are respectively derived.

[0180] Using the estimated errors ΔH_(LL), ΔH_(RL), ΔH_(LR) and ΔH_(RR)derived from the equations (17) to (20), the filter characteristics ofthe filter means 40-1, 40-2, 40-3 and 40-4 are updated per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging). For example, assuming that impulse responsesh_(LL), h_(RL), h_(LR) and h_(RR) after K-th updating are given ash_(LL)(k), h_(RL)(k), h_(LR)(k) and h_(RR)(k), using impulse responsesΔh_(LL), ΔH_(RL), Δh_(LR) and Δh_(RR) corresponding to the derivedestimated errors ΔH_(LL), ΔH_(RL), ΔH_(LR) and ΔH_(RR),

h _(LL)(k+1)=h _(LL)(k)+αΔh _(LL)   (21)

h _(RL)(k+1)=h _(RL)(k)+αΔh _(RL)   (22)

h _(LR)(k+1)=h _(LR)(k)+αΔh _(LR)   (23)

h _(RR)(k+1)=h _(RR)(k)+αΔh _(RR)   (24)

[0181] where α is a suitably set convergence coefficient.

[0182] Using these updating equations, (k+1)th impulse responsesh_(LL)(k+1), h_(RL)(k+1), h_(LR)(k+1) and h_(RR)(k+1) are derived andset to the filter means 40-1, 40-2, 40-3 and 40-4, respectively, whichis repeated per suitably determined prescribed time period (e.g. timeperiod of performing ensemble averaging).

[0183] The results of carrying out simulations using the signals x_(L)and x_(R) or the sum and difference signals x_(M) and x_(S) as referencesignals with respect to the stereo echo canceller 16, 24 of FIG. 1, areshown in FIGS. 3 to 8 for each of the audio transfer systems. As thesignals x_(L) and x_(R), stereo audio signals based on a human voicewere used. In FIGS. 3 to 8, the axis of abscissas represents the numberof blocks (one block represents a time period of performing ensembleaveraging and is set to about 2.3 seconds in the simulations), and thefilter characteristics are updated per block. The filter characteristicsare not set in the first block, the initial setting is executed in thesecond block, then updating is carried out per block. The axis ofordinates represents the estimated error (dB) of the transfer function,and the initial state where the filter characteristics are not set isdefined as 0 dB. Table 1 shows the conditions of the respectivesimulations of FIGS. 3 to 8. TABLE 1 Reference Double Change in FigureNumber Signal Talk Transfer System x_(L), x_(R) NO NO x_(M), x_(S) NO NOx_(L), x_(R) YES NO x_(M), x_(S) YES NO x_(L), x_(R) YES YES x_(M),x_(S) YES YES

[0184] In Table 1, “Double Talk” represents the state where soundsreproduced from the loudspeakers SP(L) and SP(R) as well as a voiceuttered by a person present in that room are simultaneously collected bythe microphones MC(L) and MC(R). Since the teleconferencing system isused in general in the state where the double talk occurs, it isrequired that the sufficient echo cancel performance can be achievedeven when the double talk occurs. On the other hand, in Table 1, “Changein Transfer System” represents changing the transfer functions while theestimated errors are converging, supposing a case where the microphonepositions are moved, or the like. As an operation of the echo canceller,it is required that even if the estimated error temporarily increasesdue to change in transfer function, it again go toward convergence.

[0185] The simulation results of FIGS. 3 to 8 are considered. ComparingFIGS. 3 and 4, when the signals x_(L) and x_(R) are used as referencesignals (FIG. 3), because a correlation between the signals x_(L) andx_(R) is high, estimated errors only drop to about −15 to −25 dB at mostin the 20th block (about 45 seconds) from the start of the operation. Incontrast, when the signals x_(M) and x_(S) are used as reference signals(FIG. 4), because a correlation between the signals x_(M) and x_(S) islow, estimated errors drop to about −35 to −45 dB in the 20th block fromthe start of the operation, and thus it is seen that the sufficient echocancel performance can be achieved. Comparing FIGS. 5 and 6 showingcases where the double talk exists, when the signals x_(L) and x_(R) areused as reference signals (FIG. 5), estimated errors only drop to about−10 to −20 dB at most in the 20th block from the start of the operation.In contrast, when the signals x_(M) and x_(S) are used as referencesignals (FIG. 6), estimated errors drop to about −23 to −30 dB, and thusit is seen that even if the double talk exists, the sufficient echocancel performance can be achieved. This is because, since a voiceuttered by a person present in a room is uncorrelated with soundsreproduced from the loudspeakers SP(L) and SP(R), when the transferfunction calculating means 58 calculates transfer functions of therespective systems, components of the voice uttered by the person in theroom are canceled through the foregoing cross-spectrum calculation andensemble-average calculation, so that the transfer functions of therespective systems can be derived with no influence of the double talk.Comparing FIGS. 7 and 8 showing cases where the double talk and thechange of the transfer systems are present, when the signals x_(L) andx_(R) are used as reference signals (FIG. 7), convergence of estimatederrors is poor after giving a change to the transfer systems in the 11thblock so that the estimated errors only drop to about −5 to −15 dB atmost in the 20th block. In contrast, when the signals x_(M) and x_(S)are used as reference signals (FIG. 8), convergence of estimated errorsis excellent even after the change is given to the transfer systems inthe 11th block so that the estimated errors drop to about −17 to −26 dBin the 20th block, and thus it is seen that even if the double talk andthe change of the transfer systems exist, the sufficient echo cancelperformance can be achieved.

[0186]FIG. 9 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2, wherein sum/difference signal producingmeans is arranged on transmission lines to loudspeakers. The samesymbols are used with respect to those portions common to the foregoingstructure of FIG. 1. Left/right two-channel stereo signals x_(L) andx_(R) transmitted from the spot on the counterpart side and inputtedinto line input ends LI(L) and LI(R) are inputted into sum/differencesignal producing means 52. The sum/difference signal producing means 52performs addition of the stereo signals x_(L) and x_(R) using an adder54 so as to produce a sum signal x_(M) (=x_(L)+X_(R)), while performssubtraction thereof using a subtracter 56 so as to produce a differencesignal x_(S){=x_(L)−x_(R) (or it may also be x_(R)−x_(L))}. Stereo audiosignal demodulating means 62 performs addition of the sum and differencesignals x_(M) and x_(S) using an adder 64, and further, gives thereto acoefficient ½ using a coefficient multiplier 66 to recover the originalsignal x_(L), while performs subtraction of the sum and differencesignals x_(M) and x_(S) using a subtracter 68, and further, givesthereto a coefficient ½ using a coefficient multiplier 70 to recover theoriginal signal x_(R). The recovered signals x_(L) and x_(R) areoutputted from sound output ends SO(L) and SO(R) and reproduced atloudspeakers SP(L) and SP(R), respectively.

[0187] Transfer function calculating means 58 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) produced by the sum/difference signal producingmeans 52 and signals e_(L) and e_(R) outputted from subtracters 48 and50 and, based on this cross-spectrum calculation, performs setting andupdating of filter characteristics of filter means 40-1 to 40-4.Operations thereof are the same as those described with respect to thestructure of FIG. 1. Operations of the other portions are also the sameas those described with respect to the structure of FIG. 1.

[0188]FIG. 10 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2, wherein transmission is implemented betweenthe spots A and B of FIG. 2 in the signal form of the sum signal x_(M)and the difference signal x_(S), instead of the signal form of thestereo signals x_(L) and x_(R). The same symbols are used with respectto those portions common to the foregoing structure of FIG. 1 or 9. Asum signal x_(M) (=x_(L)+x_(R)) and a difference signalx_(S){=x_(L)−x_(R) (or it may also be x_(R)−x_(L))} transmitted from thespot on the counterpart side and inputted into line input ends LI(L) andLI(R) are inputted into stereo audio signal demodulating means 62. Thestereo audio signal demodulating means 62 performs addition of the sumand difference signals x_(M) and x_(S) using an adder 64, and further,gives thereto a coefficient ½ using a coefficient multiplier 66 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 68, andfurther, gives thereto a coefficient ½ using a coefficient multiplier 70to recover the original signal x_(R). The recovered signals x_(L) andx_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively.

[0189] Transfer function calculating means 58 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) inputted from the line input ends LI(L) andLI(R) and signals e_(L) and e_(R) outputted from subtracters 48 and 50and, based on this cross-spectrum calculation, performs setting andupdating of filter characteristics of filter means 40-1 to 40-4.Operations thereof are the same as those described with respect to thestructure of FIG. 1 or 9. Sum/difference signal producing means 72performs addition, using an adder 73, of the signals e_(L) and e_(R)outputted from the subtracters 48 and 50 so as to produce a sum signale_(M)(=e_(L)+e_(R)), while performs subtraction thereof using asubtracter 75 so as to produce a difference signal e_(S) {=e_(L)−e_(R)(or it may also be e_(R)−e_(L))}, then sends them toward the spot on thecounterpart side. Operations of the other portions are the same as thosedescribed with respect to the structure of FIG. 1 or 9.

[0190]FIG. 11 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2. The same symbols are used with respect tothose portions common to the foregoing structure of FIG. 1, 9 or 10. Asum signal x_(M)(=x_(L)+x_(R)) and a difference signal x_(S){x_(L)−x_(R)(or it may also be x_(R)−x_(L)} transmitted from the spot on thecounterpart side and inputted into line input ends LI(L) and LI(R) areinputted into stereo audio signal demodulating means 62. The stereoaudio signal demodulating means 62 performs addition of the sum anddifference signals x_(M) and x_(S) using an adder 64, and further, givesthereto a coefficient ½ using a coefficient multiplier 66 to recover theoriginal signal x_(L), while performs subtraction of the sum anddifference signals x_(M) and x_(S) using a subtracter 68, and further,gives thereto a coefficient ½ using a coefficient multiplier 70 torecover the original signal x_(R). The recovered signals x_(L) and x_(R)are outputted from sound output ends SO(L) and SO(R) and reproduced atloudspeakers SP(L) and SP(R), respectively.

[0191] Sum/difference signal producing means 52 performs addition, usingan adder 54, of the stereo signals x_(L) and x_(R) recovered by thestereo audio signal demodulating means 62 so as to produce a sum signalx_(M)(=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 56 so as to produce a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}. Transfer function calculating means 58implements a cross-spectrum calculation between the sum signal x_(M) andthe difference signal x_(S) produced by the sum/difference signalproducing means 52 and signals e_(L) and e_(R) outputted fromsubtracters 48 and 50 and, based on this cross-spectrum calculation,performs setting and updating of filter characteristics of filter means40-1 to 40-4. Operations thereof are the same as those described withrespect to the structure of FIG. 1, 9 or 10. Sum/difference signalproducing means 72 performs addition, using an adder 73, of the signalse_(L) and e_(R) outputted from the subtracters 48 and 50 so as toproduce a sum signal e_(M)(=e_(L)+e_(R)), while performs subtractionthereof using a subtracter 75 so as to produce a difference signale_(S){=e_(L)−e_(R) (or it may also be e_(R)−e_(L))}, then sends themtoward the spot on the counterpart side. Operations of the otherportions are the same as those described with respect to the structureof FIG. 1, 9 or 10.

[0192]FIG. 12 shows a structural example in the stereo echo canceller16, 24. Left/right two-channel stereo signals x_(L) and x_(R)transmitted from the spot on the counterpart side and inputted into lineinput ends LI(L) and LI (R) are outputted from sound output ends SO(L)and SO(R) as they are (i.e. not through sum/difference signal producingmeans 152), and reproduced at loudspeakers SP(L) and SP(R),respectively.

[0193] The sum/difference signal producing means 152 performs addition,using an adder 154, of the left/right two-channel stereo signals x_(L)and x_(R) inputted into the line input ends LI(L) and LI(R) so as toproduce a sum signal x_(M) (=x_(L)+x_(R)), while performs subtractionthereof using a subtracter 156 so as to produce a difference signalx_(S){=x_(L)−x_(R) (or x_(R)−x_(L))}.

[0194] Filer means 140-1 to 140-4 are formed by, for example, FIRfilters. These filter means 140-1 to 140-4 are each set with an impulseresponse corresponding to a composite transfer function in the form ofcombination of transfer functions of suitable two systems among transferfunctions H_(LL), H_(LR), H_(RL) and H_(RR) of four audio transfersystems between the loudspeakers SP(L) and SP(R) and microphones MC(L)and MC(R), respectively, and perform a convolution calculation of thesum and difference signals (low-correlation composite signals) usingsuch impulse responses, thereby producing echo cancel signals EC1 toEC4, respectively.

[0195] An adder 144 performs a calculation of EC1+EC3. An adder 146performs a calculation of EC2+EC4. A subtracter 148 subtracts an echocancel signal EC1+EC3 from a collected audio signal of the microphoneMC(L) inputted from a sound input end SI(L), thereby to perform echocancellation. A subtracter 150 subtracts an echo cancel signal EC2+EC4from a collected audio signal of the microphone MC(R) inputted from asound input end SI(R), thereby to perform echo cancellation. Theseecho-canceled signals of the respective left and right channels areoutputted from line output ends LO(L) and LO(R), respectively, andtransmitted toward the spot on the counterpart side.

[0196] Transfer function calculating means 158 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) produced by the sum/difference signal producingmeans 152 and signals e_(L) and e_(R) outputted from the subtracters 148and 150 and, based on this cross-spectrum calculation, performs settingand updating of filter characteristics (impulse responses) of the filtermeans 140-1 to 140-4. Specifically, upon starting the system, the filtercharacteristics of the filter means 140-1 to 140-4 are not set, i.e.coefficients are all set to zero, so that the echo cancel signals EC1 toEC4 are zero, and thus the collected audio signals of the microphonesMC(L) and MC(R) themselves are outputted from the subtracters 148 and150. Therefore, at this time, the transfer function calculating means158 performs the cross-spectrum calculation between the sum signal x_(M)and the difference signal x_(S) produced by the sum/difference signalproducing means 152 and the collected audio signals e_(L) and e_(R) ofthe microphones MC(L) and MC(R) outputted from the subtracters 148 and150 and, based on this cross-spectrum calculation, derives a pluralityof composite transfer functions each in the form of combination oftransfer functions of suitable two systems among transfer functionsH_(LL), H_(LR), H_(RL) and H_(RR) of four audio transfer systems betweenthe loudspeakers SP(L) and SP(R) and the microphones MC(L) and MC(R),respectively, and implements initial setting of the filtercharacteristics of the filter means 140-1 to 140-4 to valuescorresponding to such composite transfer functions. After the initialsetting, since the echo cancel signals are produced by the filter means140-1 to 140-4, the echo cancel error signals e_(L) and e_(R)corresponding to difference signals between the collected audio signalsof the microphones MC(L) and MC(R) and the echo cancel signals EC1 toEC4 are outputted from the subtracters 148 and 150. Therefore, at thistime, the transfer function calculating means 158 performs thecross-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) produced by the sum/difference signal producingmeans 152 and the echo cancel error signals e_(L) and e_(R) outputtedfrom the subtracters 148 and 150 and, based on this cross-spectrumcalculation, derives estimated errors of the foregoing compositetransfer functions, respectively, and updates the filter characteristicsof the filter means 140-1 to 140-4 to values that cancel such estimatederrors, respectively. By repeating this updating operation perprescribed time period, the echo cancel error can be converged to aminimum value. Further, even if the transfer functions change due tomovement of the microphone positions or the like, the echo cancel errorcan be converged to a minimum value by sequentially updating the filtercharacteristics of the filter means 140-1 to 140-4 depending thereon.

[0197] Correlation detecting means 160 detects a correlation between thesum signal x_(M) and the difference signal x_(S) based on a correlationvalue calculation or the like, and stops updating of the foregoingfilter characteristics when the correlation value is no less than aprescribed value. When the correlation value becomes lower than theprescribed value, updating of the foregoing filter characteristics isrestarted.

[0198] Herein, the filter characteristics (impulse responses) that areset to the filter means 140-1 to 140-4 by the transfer functioncalculating means 158 will be described. In the transfer functioncalculating means 158, the following calculation is performed.

[0199] (In Case of Fixed Type Operation)

[0200] The signals x_(L) and x_(R) are

x _(L)=(x _(M) +x _(S))/2

x _(R)=(x _(M) −x _(S))/2

[0201] hence, output signals Y_(L) and Y_(R) of the microphones MC(L)and MC(R) become $\begin{matrix}\begin{matrix}{Y_{L} = {{\left( {X_{M} + X_{S}} \right){H_{LL}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RL}/2}}}} \\{= {{X_{M}\left\{ {\left( {H_{LL} + H_{RL}} \right)/2} \right\}} + {X_{S}\left\{ {\left( {H_{LL} - H_{RL}} \right)/2} \right\}}}}\end{matrix} & (25) \\\begin{matrix}{Y_{R} = {{\left( {X_{M} + X_{S}} \right){H_{LR}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RR}/2}}}} \\{= {{X_{M}\left\{ {\left( {H_{LR} + H_{RR}} \right)/2} \right\}} + {X_{S}{\left\{ {\left( {H_{LR} - H_{RR}} \right)/2} \right\}.}}}}\end{matrix} & (26)\end{matrix}$

[0202] When the composite transfer functions are given as

H _(ML)=(H _(LL) +H _(RL))/2

H _(SL)=(H _(LL) −H _(RL))/2

H _(MR)=(H _(LR) +H _(RR))/2

H _(SR)=(H _(LR) −H _(RR))/2

[0203] the equations (25) and (26) respectively become

Y _(L) =X _(M) ·H _(ML) +X _(S) ·H _(SL)   (25′)

Y _(R) =X _(M) ·H _(MR) +X _(S) ·H _(SR)   (26′).

[0204] When both sides of the equations (25′) and (26′) are multipliedby complex conjugates X_(M)* and X_(S)* of X_(M) and X_(S) andensemble-averaged,

ΣX _(M) *·Y _(L) =ΣX _(M) *·X _(M) ·H _(ML) +ΣX _(M) *·X _(S) ·H _(SL)  (27)

ΣX _(S) *·Y _(L) =ΣX _(S) *·X _(M) ·H _(ML) +ΣX _(S) *·X _(S) ·H _(SL)  (28)

ΣX _(M) *·Y _(R) =ΣX _(M) *·X _(M) ·H _(MR) +ΣX _(M) *·X _(S) ·H _(SR)  (29)

ΣX _(S) *·Y _(R) =ΣX _(S) *·X _(M) ·H _(MR) +ΣX _(S) *·X _(S) ·H _(SR)  (30)

[0205] are respectively obtained.

[0206] In the equations (27) to (30), since X_(M) and X_(S) areapproximately uncorrelated with each other, such a term havingX_(M)*·X_(S) or X_(S)*·X_(M) becomes approximately zero whenensemble-averaged. Further,

X _(M) *·X _(M) =|X _(M)|²

X _(S) *·X _(S) =|X _(S)|²

[0207] hence, the equations (27) to (30) respectively become

ΣX _(M) *·Y _(L) =Σ|X _(M)|² H _(ML)   (27′)

ΣX _(S) *·Y _(L) =Σ|X _(S)|² H _(SL)   (28′)

ΣX _(M) *·Y _(R) =Σ|X _(M)|² H _(MR)   (29′)

ΣX _(S) *·Y _(R) =Σ|X _(S)|² H _(SR)   (30′).

[0208] From the equations (27′) to (30′),

H _(ML) =ΣX _(M) *·Y _(L) /Σ|X _(M)|²   (31)

H _(SL) =ΣX _(X) *·Y _(L) /Σ|X _(S)|²   (32)

H _(MR) =ΣX _(M) *·Y _(R) /Σ|X _(M)|²   (33)

H _(SR) =ΣX _(S) *·Y _(R) /Σ|X _(S)|²   (34)

[0209] are respectively derived.

[0210] Impulse responses h_(ML), h_(SL), h_(MR) and h_(SR) obtained byapplying the inverse Fourier transformation to these derived compositetransfer functions H_(ML), H_(SL), H_(MR) and H_(SR) are the filtercharacteristics to be set to the filter means 140-1, 140-2, 140-3 and140-4, respectively. Therefore, the transfer function calculating means158 derives the respective composite transfer functions H_(ML), H_(SL),H_(MR) and H_(SR) from the equations (31) to (34) based on the sumsignal x_(M), the difference signal x_(S), and output signals y_(L) andy_(R) of the microphones MC(L) and MC(R) that are inputted, derives theimpulse responses h_(ML), h_(SL), h_(MR) and h_(SR) by applying theinverse Fourier transformation to those derived composite transferfunctions, sets the derived impulse responses to the filter means 140-1,140-2, 140-3 and 140-4, respectively, and further, updates the impulseresponses by repeating this calculation per suitably determinedprescribed time period (e.g. time period of performing ensembleaveraging).

[0211] (In Case of Adaptive Type Operation)

[0212] Assuming that the filter characteristics set to the filter means140-1, 140-2, 140-3 and 140-4 are given as H{circumflex over ( )}_(ML),H{circumflex over ( )}_(SL), H{circumflex over ( )}_(MR) andH{circumflex over ( )}_(SR) (h{circumflex over ( )}_(ML), h{circumflexover ( )}_(SL), h{circumflex over ( )}_(MR) and h{circumflex over( )}_(SR) when expressed in terms of the impulse responses), the signalse_(L) and e_(R) outputted from the subtracters 148 and 150 become$\begin{matrix}\begin{matrix}{E_{L} = {Y_{L} - \left( {{X_{M} \cdot H_{ML}^{\hat{}}} + {X_{S} \cdot H_{SL}^{\hat{}}}} \right)}} \\{= {\left\{ {{\left( {X_{M} + X_{S}} \right){H_{LL}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RL}/2}}} \right\} -}} \\{\left( {{X_{M} \cdot H_{ML}^{\hat{}}} + {X_{S} \cdot H_{SL}^{\hat{}}}} \right)} \\{= {{X_{M}\left\{ {{\left( {H_{LL} + H_{RL}} \right)/2} - H_{ML}^{\hat{}}} \right\}} +}} \\{{X_{S}\left\{ {{\left( {H_{LL} - H_{RL}} \right)/2} - H_{SL}^{\hat{}}} \right\}}}\end{matrix} & (35) \\\begin{matrix}{E_{R} = {Y_{R} - \left( {{X_{M} \cdot H_{MR}^{\hat{}}} + {X_{S} \cdot H_{SR}^{\hat{}}}} \right)}} \\{= {\left\{ {{\left( {X_{M} + X_{S}} \right){H_{LR}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RR}/2}}} \right\} -}} \\{\left( {{X_{M} \cdot H_{MR}^{\hat{}}} + {X_{S} \cdot H_{SR}^{\hat{}}}} \right)} \\{= {{X_{M}\left\{ {{\left( {H_{LR} + H_{RR}} \right)/2} - H_{MR}^{\hat{}}} \right\}} +}} \\{{X_{S}{\left\{ {{\left( {H_{LR} - H_{RR}} \right)/2} - H_{SR}^{\hat{}}} \right\}.}}}\end{matrix} & (36)\end{matrix}$

[0213] When the composite transfer functions are given as

H _(ML)=(H _(LL) +H _(RL))/2

H _(SL)=(H _(LL) −H _(RL))/2

H _(MR)=(H _(LR) +H _(RR))/2

H _(SR)=(H _(LR) −H _(RR))/2

[0214] the equations (35) and (36) respectively become

E _(L) =X _(M)(H _(ML) −H{circumflex over ( )} _(ML))+X _(S)(H _(SL)−H{circumflex over ( )} _(SL))   (35′)

E _(R) =X _(M)(H _(MR) −H{circumflex over ( )} _(MR))+X _(S)(H _(SR)−H{circumflex over ( )} _(SR) )   (36′).

[0215] When the estimated errors of the composite transfer functions aregiven as

ΔH _(ML) =H _(ML) −H{circumflex over ( )} _(ML)

ΔH _(SL) =H _(SL) −H{circumflex over ( )} _(SL)

ΔH _(MR) =H _(MR) −H{circumflex over ( )} _(MR)

ΔH _(SR) =H _(SR) −H{circumflex over ( )} _(SR)

[0216] the equations (35′) and (36′) respectively become

E _(L) =X _(M) ·ΔH _(ML) +X _(S) ·ΔH _(SL)   (35″)

E _(R) =X _(M) ·ΔH _(MR) +X _(S) ·ΔH _(SR)   (36″).

[0217] When both sides of the equations (35″) and (36″) are multipliedby complex conjugates X_(M)* and X_(S)* of X_(M) and X_(S) andensemble-averaged,

ΣX _(M) *·E _(L) =ΣX _(M) *·X _(M) ·ΔH _(ML) +ΣX _(M) *·X _(S) ·ΔH _(SL)  (37)

ΣX _(S) *·E _(L) =ΣX _(S) *·X _(M) ·ΔH _(ML) +ΣX _(S) *·X _(S) ·ΔH _(SL)  (38)

ΣX _(M) *·E _(R) =ΣX _(M) *·X _(M) ·ΔH _(MR) +ΣX _(M) *·X _(S) ·ΔH _(SR)  (39)

ΣX _(S) *·E _(R) =ΣX _(S) *·X _(M) ·ΔH _(MR) +ΣX _(S) *·X _(S) ·ΔH _(SR)  (40)

[0218] are respectively obtained.

[0219] In the equations (37) to (40), since X_(M) and X_(S) areapproximately uncorrelated with each other, such a term havingX_(M)*·X_(S) or X_(S)*·X_(M) becomes approximately zero whenensemble-averaged. Further,

X _(M) *·X _(M) =|X _(M)|²

X _(S) *·X _(S) =|X _(S)|²

[0220] hence, the equations (37) to (40) respectively become

ΣX _(M) *·E _(L) =Σ|X _(M)|² ΔH _(ML)   (37′)

ΣX _(S) *·E _(L) =Σ|X _(S)|² ΔH _(SL)   (38′)

ΣX _(M) *·E _(R) =Σ|X _(M)|² ΔH _(MR)   (39′)

ΣX _(S) *·E _(R) =Σ|X _(S)|² ΔH _(SR)   (40′).

[0221] From the equations (37′) to (40′),

ΔH _(ML) =ΣX _(M) *·E _(L) /Σ|X _(M)|²   (41)

ΔH _(SL) =ΣX _(S) *·E _(L) /Σ|X _(S)|²   (42)

ΔH _(MR) =ΣX _(M) *·E _(R) /Σ|X _(M)|²   (43)

ΔH _(SR) =ΣX _(S) *·E _(R) /Σ|X _(S)|²   (44)

[0222] are respectively derived.

[0223] Using the estimated errors ΔH_(ML), ΔH_(SL), ΔH_(MR) and ΔH_(SR)derived from the equations (40) to (44), the filter characteristics ofthe filter means 140-1, 140-2, 140-3 and 140-4 are updated per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging). For example, assuming that impulse responsesh_(ML), h_(SL), h_(MR) and h_(SR) after K-th updating are given ash_(ML)(k), h_(SL)(k), h_(MR)(k) and h_(SR)(k) , using impulse responsesΔh_(ML), Δh_(SL), Δh_(MR) and Δh_(SR) corresponding to the derivedestimated errors ΔH_(ML), ΔH_(SL), ΔH_(MR) and ΔH_(SR),

h _(ML)(k+1)=h _(ML)(k)+αΔh _(ML)   (45)

h _(SL)(k+1)=h _(SL)(k)+αΔh _(SL)   (46)

h _(MR)(k+1)=h _(MR)(k)+αΔh _(MR)   (47)

h _(SR)(k+1)=h _(SR)(k)+αΔh _(SR)   (48).

[0224] Using these updating equations, (k+1)th impulse responsesh_(ML)(k+1), h_(SL)(k+1), h_(MR)(k+1) and h_(SR)(k+1) are derived andset to the filter means 140-1, 140-2, 140-3 and 140-4, respectively,which is repeated per suitably determined prescribed time period (e.g.time period of performing ensemble averaging).

[0225]FIG. 13 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2, wherein sum/difference signal producingmeans is arranged on transmission lines to loudspeakers. The samesymbols are used with respect to those portions common to the foregoingstructure of FIG. 12. Left/right two-channel stereo signals x_(L) andx_(R) transmitted from the spot on the counterpart side and inputtedinto line input ends LI(L) and LI(R) are inputted into sum/differencesignal producing means 152. The sum/difference signal producing means152 performs addition of the stereo signals x_(L) and x_(R) using anadder 154 so as to produce a sum signal x_(M) (=x_(L)+x_(R)), whileperforms subtraction thereof using a subtracter 156 so as to produce adifference signal x_(S) {=x_(L)−x_(R) (or it may also be x_(R)−x_(L))}.Stereo audio signal demodulating means 162 performs addition of the sumand difference signals x_(M) and x_(S) using an adder 164, and further,gives thereto a coefficient 1/2 using a coefficient multiplier 166 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 168, andfurther, gives thereto a coefficient 1/2 using a coefficient multiplier170 to recover the original signal x_(R). The recovered signals x_(L)and x_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively.

[0226] Transfer function calculating means 158 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) produced by the sum/difference signal producingmeans 152 and signals e_(L) and e_(R) outputted from subtracters 148 and150 and, based on this cross-spectrum calculation, performs setting andupdating of filter characteristics of filter means 140-1 to 140-4.Operations thereof are the same as those described with respect to thestructure of FIG. 12. Operations of the other portions are also the sameas those described with respect to the structure of FIG. 12.

[0227]FIG. 14 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2, wherein transmission is implemented betweenthe spots A and B of FIG. 2 in the signal form of the sum signal x_(M)and the difference signal x_(X), instead of the signal form of thestereo signals x_(L) and x_(R). The same symbols are used with respectto those portions common to the foregoing structure of FIG. 12 or 13. Asum signal x_(M) (=x_(L)+x_(R)) and a difference signal x_(S){=x_(L)−x_(R) (or it may also be x_(R)−x_(L))} transmitted from the spoton the counterpart side and inputted into line input ends LI(L) andLI(R) are inputted into stereo audio signal demodulating means 162. Thestereo audio signal demodulating means 162 performs addition of the sumand difference signals x_(M) and x_(S) using an adder 164, and further,gives thereto a coefficient 1/2 using a coefficient multiplier 166 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 168, andfurther, gives thereto a coefficient 1/2 using a coefficient multiplier170 to recover the original signal x_(R). The recovered signals x_(L)and x_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively.

[0228] Transfer function calculating means 158 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) inputted from the line input ends LI(L) andLI(R) and signals e_(L) and e_(R) outputted from subtracters 148 and 150and, based on this cross-spectrum calculation, performs setting andupdating of filter characteristics of filter means 140-1 to 140-4.Operations thereof are the same as those described with respect to thestructure of FIG. 12 or 13. Sum/difference signal producing means 172performs addition, using an adder 173, of the signals e_(L) and e_(R)outputted from the subtracters 148 and 150 so as to produce a sum signale_(M) (=e_(L)+e_(R)), while performs subtraction thereof using asubtracter 175 so as to produce a difference signal e_(S) {=e_(L)−e_(R)(or it may also be e_(R)−e_(L))}, then sends them toward the spot on thecounterpart side. Operations of the other portions are the same as thosedescribed with respect to the structure of FIG. 12 or 13.

[0229]FIG. 15 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2. The same symbols are used with respect tothose portions common to the foregoing structure of FIG. 12, 13 or 14. Asum signal x_(M) (=x_(L)+x_(R)) and a difference signalx_(S){=x_(L)−x_(R) (or it may also be x_(R)−x_(L))} transmitted from thespot on the counterpart side and inputted into line input ends LI(L) andLI(R) are inputted into stereo audio signal demodulating means 162. Thestereo audio signal demodulating means 162 performs addition of the sumand difference signals x_(M) and x_(S) using an adder 164, and further,gives thereto a coefficient 1/2 using a coefficient multiplier 166 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 168, andfurther, gives thereto a coefficient 1/2 using a coefficient multiplier170 to recover the original signal x_(R). The recovered signals x_(L)and x_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively.

[0230] Sum/difference signal producing means 152 performs addition,using an adder 154, of the stereo signals x_(L) and x_(R) recovered bythe stereo audio signal demodulating means 162 so as to produce a sumsignal x_(M) (=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 156 so as to produce a difference signal x_(S) {=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}. Transfer function calculating means158 implements a cross-spectrum calculation between the sum signal x_(M)and the difference signal x_(S) produced by the sum/difference signalproducing means 152 and signals e_(L) and e_(R) outputted fromsubtracters 148 and 150 and, based on this cross-spectrum calculation,performs setting and updating of filter characteristics of filter means140-1 to 140-4. Operations thereof are the same as those described withrespect to the structure of FIG. 12, 13 or 14. Sum/difference signalproducing means 172 performs addition, using an adder 173, of thesignals e_(L) and e_(R) outputted from the subtracters 148 and 150 so asto produce a sum signal e_(M) (=e_(L)+e_(R)), while performs subtractionthereof using a subtracter 175 so as to produce a difference signale_(S) {=e_(L)−e_(R) (or it may also be e_(R)−e_(L))}, then sends themtoward the spot on the counterpart side. Operations of the otherportions are the same as those described with respect to the structureof FIG. 12, 13 or 14.

[0231]FIG. 16 shows another structural example in the stereo echocanceller 16, 24. A sum signal x_(M) (=x_(L)+x_(R)) and a differencesignal x_(S){=x_(L)−x_(R) (or it may also be x_(R)−x_(L))} transmittedfrom the spot on the counterpart side and inputted into line input endsLI(L) and LI(R) are inputted into stereo audio signal demodulating means262. The stereo audio signal demodulating means 262 performs addition ofthe sum and difference signals x_(M) and x_(S) using an adder 264, andfurther, gives thereto a coefficient 1/2 using a coefficient multiplier266 to recover the original signal x_(L), while performs subtraction ofthe sum and difference signals x_(M) and x_(S) using a subtracter 268,and further, gives thereto a coefficient 1/2 using a coefficientmultiplier 270 to recover the original signal x_(R). The recoveredsignals x_(L) and x_(R) are outputted from sound output ends SO(L) andSO(R) and reproduced at loudspeakers SP(L) and SP(R), respectively.

[0232] Collected audio signals y_(L) and y_(R) of microphones MC(L) andMC(R) are inputted into sum/difference signal producing means 272. Thesum/difference signal producing means 272 implements addition of themicrophone collected audio signals y_(L) and y_(R) using an adder 273 soas to produce a sum signal y_(M), while implements subtraction thereofusing a subtracter 275 so as to produce a difference signal y_(S).

[0233] Filer means 240-1 to 240-4 are formed by, for example, FIRfilters. These filter means 240-1 to 240-4 are each set with an impulseresponse corresponding to a composite transfer function in the form ofcombination of transfer functions of suitable two systems among transferfunctions H_(LL), H_(LR), H_(RL) and H_(RR) of four audio transfersystems between the loudspeakers SP(L) and SP(R) and microphones MC(L)and MC(R), respectively, and perform a convolution calculation of theleft/right two-channel stereo signals x_(L) and x_(R) using such impulseresponses, thereby producing echo cancel signals EC1 to EC4,respectively.

[0234] An adder 244 performs a calculation of EC1+EC3. An adder 246performs a calculation of EC2+EC4. A subtracter 248 subtracts an echocancel signal EC1+EC3 from the sum signal y_(M), thereby to perform echocancellation. A subtracter 250 subtracts an echo cancel signal EC2+EC4from the difference signal y_(S), thereby to perform echo cancellation.Signals e_(M) and e_(S) outputted from the subtracters 248 and 250 areoutputted from line output ends LO(L) and LO(R), respectively, andtransmitted toward the spot on the counterpart side.

[0235] Transfer function calculating means 258 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) inputted from the line input ends LI(L) andLI(R) and the signals e_(M) and e_(S) outputted from the subtracters 248and 250 and, based on this cross-spectrum calculation, performs settingand updating of filter characteristics (impulse responses) of the filtermeans 240-1 to 240-4. Specifically, upon starting the system, the filtercharacteristics of the filter means 240-1 to 240-4 are not set, i.e.coefficients are all set to zero, so that the echo cancel signals EC1 toEC4 are zero, and thus the sum signal y_(M) and the difference signaly_(S) outputted from the sum/difference signal producing means 272, asthey are, are outputted from the subtracters 248 and 250. Therefore, atthis time, the transfer function calculating means 258 performs thecross-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) inputted from the line input ends LI(L) andLI(R) and the sum signal y_(M) and the difference signal y_(S) outputtedfrom the subtracters 248 and 250 and, based on this cross-spectrumcalculation, derives a plurality of composite transfer functions each inthe form of combination of transfer functions of suitable two systemsamong transfer functions H_(LL), H_(LR), H_(RL) and H_(RR) of four audiotransfer systems between the loudspeakers SP(L) and SP(R) and themicrophones MC(L) and MC(R), respectively, and implements initialsetting of the filter characteristics of the filter means 240-1 to 240-4to values corresponding to such composite transfer functions. After theinitial setting, since the echo cancel signals are produced by thefilter means 240-1 to 240-4, the echo cancel error signals e_(M) ande_(S) corresponding to difference signals between the sum signal y_(M)and the difference signal y_(S) outputted from the sum/difference signalproducing means 272 and the echo cancel signals EC1 to EC4 are outputtedfrom the subtracters 248 and 250. Therefore, at this time, the transferfunction calculating means 258 performs the cross-spectrum calculationbetween the sum signal x_(M) and the difference signal x_(S) inputtedfrom the line input ends LI(L) and LI(R) and the echo cancel errorsignals e_(M) and e_(S) outputted from the subtracters 248 and 250 and,based on this cross-spectrum calculation, derives estimated errors ofthe foregoing composite transfer functions, respectively, and updatesthe filter characteristics of the filter means 240-1 to 240-4 to valuesthat cancel such estimated errors, respectively. By repeating thisupdating operation per prescribed time period, the echo cancel error canbe converged to a minimum value. Further, even if the transfer functionschange due to movement of the microphone positions or the like, the echocancel error can be converged to a minimum value by sequentiallyupdating the filter characteristics of the filter means 240-1 to 240-4depending thereon.

[0236] Correlation detecting means 260 detects a correlation between thesum signal x_(M) and the difference signal x_(S) based on a correlationvalue calculation or the like, and stops updating of the foregoingfilter characteristics when the correlation value is no less than aprescribed value. When the correlation value becomes lower than theprescribed value, updating of the foregoing filter characteristics isrestarted.

[0237] Herein, the filter characteristics (impulse responses) that areset to the filter means 240-1 to 240-4 by the transfer functioncalculating means 258 will be described. In the transfer functioncalculating means 258, the following calculation is performed.

[0238] (In Case of Fixed Type Operation)

[0239] The sum and difference signals y_(M) and y_(S) of the outputsignals y_(L) and y_(R) of the microphones MC(L) and MC(R), assumingthat frequency-axis expressions of y_(M) and y_(S) are respectivelygiven as y_(M) and y_(S), become $\begin{matrix}\begin{matrix}{Y_{M} = {Y_{L} + Y_{R}}} \\{= {\left( {{X_{L} \cdot H_{LL}} + {X_{R} \cdot H_{RL}}} \right) + \left( {{X_{L} \cdot H_{LR}} + {X_{R} \cdot H_{RR}}} \right)}} \\{= {{X_{L}\left( {H_{LL} + H_{LR}} \right)} + {X_{R}\left( {H_{RL} + H_{RR}} \right)}}}\end{matrix} & (49) \\\begin{matrix}{Y_{S} = {Y_{L} - Y_{R}}} \\{= {\left( {{X_{L} \cdot H_{LL}} + {X_{R} \cdot H_{RL}}} \right) - \left( {{X_{L} \cdot H_{LR}} + {X_{R} \cdot H_{RR}}} \right)}} \\{= {{X_{L}\left( {H_{LL} - H_{LR}} \right)} + {{X_{R}\left( {H_{RL} - H_{RR}} \right)}.}}}\end{matrix} & {(50).}\end{matrix}$

[0240] When the composite transfer functions are given as

H _(LM) =H _(LL) +H _(LR)

H _(RM) =H _(RL) +H _(RR)

H _(LS) =H _(LL) −H _(LR)

H _(RS) =H _(RL) −H _(RR)

then

X _(L)=(X _(M) +X _(S))/2

X _(R)=(X _(M) −X _(S))/2

[0241] hence, the equations (49) and (50) respectively become$\begin{matrix}{Y_{M} = {{\left( {X_{M} + X_{S}} \right){H_{LM}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RM}/2}}}} \\{= {{{X_{M}\left( {H_{LM} + H_{RM}} \right)}/2} + {{X_{S}\left( {H_{LM} - H_{RM}} \right)}/2}}} \\{Y_{S} = {{\left( {X_{M} + X_{S}} \right){H_{LS}/2}} + {\left( {X_{M} - X_{S}} \right){H_{RS}/2}}}} \\{= {{{X_{M}\left( {H_{LS} + H_{RS}} \right)}/2} + {{X_{S}\left( {H_{LS} - H_{RS}} \right)}/2}}}\end{matrix}$

[0242] thus

2Y _(M) =X _(M)(H _(LM) +H _(RM))+X _(S)(H _(LM) −H _(RM))   (49′)

2Y _(S) =X _(M)(H _(LS) +H _(RS))+X _(S)(H _(LS) −H _(RS))   (50′)

[0243] When both sides of the equations (49′) and (50′) are multipliedby complex conjugates X_(M)* and X_(S)* of X_(M) and X_(S) andensemble-averaged,

ΣX _(M) *·Y _(M) =ΣX _(M) *·X _(M)(H _(LM) +H _(RM))+ΣX _(M) *·X_(S)(H_(LM) −H _(RM))   (51)

ΣX _(S) *·Y _(M) =ΣX _(S) *·X _(M)(H _(LM) +H _(RM))+ΣX _(S) *·X_(S)(H_(LM) −H _(RM))   (52)

ΣX _(M) *·Y _(S) =ΣX _(M) *·X _(M)(H _(LS) +H _(RS))+ΣX _(M) *·X_(S)(H_(LS) −H _(RS))   (53)

ΣX _(S) *·Y _(S) =ΣX _(S) *·X _(M)(H _(LS) +H _(RS))+ΣX _(M) *·X_(S)(H_(LS) −H _(RS))   (54)

[0244] are respectively obtained.

[0245] In the equations (51) to (54), since X_(M) and X_(S) areapproximately uncorrelated with each other, such a term havingX_(M)*·X_(S) or X_(S)*·X_(M) becomes approximately zero whenensemble-averaged. Further,

X _(M) *·X _(M) =|X _(M)|²

X _(S) *·X _(S) =|X _(S)|²

[0246] hence, the equations (51) to (54) respectively become

ΣX _(M) *·Y _(M) =Σ|X _(M)|²(H _(LM) +H _(RM))   (51′)

ΣX _(S) *·Y _(M) =Σ|X _(S)|²(H _(LM) −H _(RM))   (52′)

ΣX _(M) *·Y _(S) =Σ|X _(M)|²(H _(LS) +H _(RS))   (53′)

ΣX _(S) *·Y _(S) =Σ|X _(S)|²(H _(LS) −H _(RS))   (54′).

[0247] By transforming the equations (51′) to (54′), the followingcomposite transfer functions are respectively derived.

H _(LM) +H _(RM) =ΣX _(M) *·Y _(M) /Σ|X _(M)|²   (51″)

H _(LM) −H _(RM) =ΣX _(S) *·Y _(M) /Σ|X _(S)|²   (52″)

H _(LS) +H _(RS) =ΣX _(M) *·Y _(S) /Σ|X _(M)|²   (53″)

H _(LS) −H _(RS) =ΣX _(S) *·Y _(S) /Σ|X _(S)|²   (54″)

[0248] From the equations (51″) to (54″),

H _(LM) =ΣX _(M) *·Y _(M) /Σ|X _(M)|² +ΣX _(S) *·Y _(M) /Σ|X _(S)|²  (55)

H _(RM) =ΣX _(M) *·Y _(M) /Σ|X _(M)|² −ΣX _(S) *·Y _(M) /Σ|X _(S)|²  (56)

H _(LS) =ΣX _(M) *·Y _(S) /Σ|X _(M)|² +ΣX _(S) *·Y _(S) /Σ|X _(S)|²  (57)

H _(RS) =ΣX _(M) *·Y _(S) /Σ|X _(M)|² −ΣX _(S) *·Y _(S) /Σ|X _(S)|²  (58)

[0249] are respectively derived.

[0250] Impulse responses h_(LM), h_(RM), h_(LS) and h_(RS) obtained byapplying the inverse Fourier transformation to these derived compositetransfer functions H_(LM), H_(RM), H_(LS) and H_(RS) are the filtercharacteristics to be set to the filter means 240-1, 240-2, 240-3 and240-4, respectively. Therefore, the transfer function calculating means258 derives the respective composite transfer functions H_(LM), H_(RM),H_(LS) and H_(RS) from the equations (55) to (58) based on the sumsignal x_(M) and the difference signal x_(S) inputted into the lineinput ends LI(L) and LI(R) and the sum signal y_(M) and the differencesignal y_(S) outputted from the sum/difference signal producing means272, derives the impulse responses h_(LM), h_(RM), h_(LS) and h_(RS) byapplying the inverse Fourier transformation to those derived compositetransfer functions, sets the derived impulse responses to the filtermeans 240-1, 240-2, 240-3 and 240-4, respectively, and further, updatesthe impulse responses by repeating this calculation per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging).

[0251] (In Case of Adaptive Type Operation)

[0252] The signals e_(M) and e_(S) outputted from the subtracters 248and 250, assuming that frequency-axis expressions of e_(M) and e_(S) arerespectively given as E_(M) and E_(S) and the filter characteristics setto the filter means 240-1, 240-2, 240-3 and 240-4 are given asH{circumflex over ( )}_(LM), H{circumflex over ( )}_(RM), H{circumflexover ( )}_(LS) and H{circumflex over ( )}_(RS) (h{circumflex over( )}_(LM), h{circumflex over ( )}_(RM), h{circumflex over ( )}_(LS) andh{circumflex over ( )}_(RS) when expressed in terms of the impulseresponses), become $\begin{matrix}\begin{matrix}{E_{M} = {\left\{ {{L\left( {H_{LL} + H_{LR}} \right)} + {X_{R}\left( {H_{RL} + H_{RR}} \right)}} \right\} -}} \\{\left( {{X_{L} \cdot H_{LM}^{\hat{}}} + {X_{R} \cdot H_{RM}^{\hat{}}}} \right)} \\{= {{L\left\{ {\left( {H_{LL} + H_{LR}} \right) - H_{LM}^{\hat{}}} \right\}} + {X_{R}\left\{ {\left( {H_{RL} + H_{RR}} \right) - H_{RM}^{\hat{}}} \right\}}}}\end{matrix} & (59) \\\begin{matrix}{E_{S} = {\left\{ {{L\left( {H_{LL} - H_{LR}} \right)} + {X_{R}\left( {H_{RL} - H_{RR}} \right)}} \right\} -}} \\{\left( {{X_{L} \cdot H_{LS}^{\hat{}}} + {X_{R} \cdot H_{RS}^{\hat{}}}} \right)} \\{= {{L\left\{ {\left( {H_{LL} - H_{LR}} \right) - H_{LS}^{\hat{}}} \right\}} + {X_{R}\left\{ {\left( {H_{RL} - H_{RR}} \right) - H_{RS}^{\hat{}}} \right\}}}}\end{matrix} & (60)\end{matrix}$

[0253] When the composite transfer functions are given as

H _(LM) =H _(LL) +H _(LR)

H _(RM) =H _(RL) +H _(RR)

H _(LS) =H _(LL) −H _(LR)

H _(RS) =H _(RL) −H _(RR)

then

X _(L)=(X _(M) +X _(S))/2

X _(R)=(X _(M) −X _(S))/2

[0254] hence, the equations (59) and (60) respectively become

E _(M)=(X _(M) +X _(S))·(H _(LM) −H{circumflex over ( )} _(LM))/2+(X_(M) −X _(S))·(H _(RM) −H{circumflex over ( )} _(RM))/2   (61)

E _(S)=(X _(M) +X _(S))·(H _(LS) −H{circumflex over ( )} _(LS))/2+(X_(M) −X _(S))·(H _(RS) −H{circumflex over ( )} _(RS))/2   (62).

[0255] When the estimated errors of the composite transfer functions aregiven as

ΔH _(LM) =H _(LM) −H{circumflex over ( )} _(LM)

ΔH _(RM) =H _(RM) −H{circumflex over ( )} _(RM)

ΔH _(LS) =H _(LS) −H{circumflex over ( )} _(LS)

ΔH _(RS) =H _(RS) −H{circumflex over ( )} _(RS)

[0256] the equations (61) and (62) respectively become

E _(M) =X _(M)(ΔH _(LM) +ΔH _(RM))/2+X _(S)(ΔH _(LM) −ΔH _(RM))/2

E _(S) =X _(M)(ΔH _(LS) +ΔH _(RS))/2+X _(S)(ΔH _(LS) −ΔH _(RS))/2

hence

2E _(M) =X _(M)(ΔH _(LM) +ΔH _(RM))+X _(S)(ΔH _(LM) −ΔH _(RM))   (61′)

2E _(S) =X _(M)(ΔH _(LS) +ΔH _(RS))+X _(S)(ΔH _(LS) −ΔH _(RS))   (62′).

[0257] When both sides of the equations (61′) and (62′) are multipliedby complex conjugates X_(M)* and X_(S)* of X_(M) and X_(S) andensemble-averaged,

ΣX _(M)*·2E _(M) =ΣX _(M) *·X _(M)(ΔH _(LM) +ΔH _(RM))+ΣX _(M) *·X_(S)(ΔH _(LM) −ΔH _(RM))   (63)

ΣX _(S)*·2E _(M) =ΣX _(S) *·X _(M)(ΔH _(LM) +ΔH _(RM))+ΣX _(S) *·X_(S)(ΔH _(LM) −ΔH _(RM))   (64)

ΣX _(M)*·2E _(S) =ΣX _(M) *·X _(M)(ΔH _(LS) +ΔH _(RS))+ΣX _(M) *·X_(S)(ΔH _(LS) −ΔH _(RS))   (65)

ΣX _(S)*·2E _(S) =ΣX _(S) *·X _(M)(ΔH _(LS) +ΔH _(RS))+ΣX _(S) *·X_(S)(ΔH _(LS) −ΔH _(RS))   (66)

[0258] are respectively obtained.

[0259] In the equations (63) to (66), since X_(M) and X_(S) areapproximately uncorrelated with each other, such a term havingX_(M)*·X_(S) or X_(S)*·X_(M) becomes approximately zero whenensemble-averaged. Further,

X _(M) *·X _(M) =|X _(M)|²

X _(S) *·X _(S) =|X _(S)|²

[0260] hence, the equations (63) to (66) respectively become

ΣX _(M)*·2E _(M) =Σ|X _(M)|²(ΔH _(LM) +H _(RM))   (63′)

ΣX _(S)*·2E _(M) =Σ|X _(S)|²(ΔH _(LM) −H _(RM))   (64′)

ΣX _(M)*·2E _(S) =Σ|X _(M)|²(ΔH _(LS) +H _(RS))   (65′)

ΣX _(S)*·2E _(S) =Σ|X _(S)|²(ΔH _(LS) −H _(RS))   (66′).

[0261] By transforming the equations (63′) to (66′), the followingcomposite transfer functions are respectively derived.

ΔH _(LM) +ΔH _(RM) =ΣX _(M)*·2E _(M) /Σ|X _(M)|²   (63″)

ΔH _(LM) −ΔH _(RM) =ΣX _(S)*·2E _(M) /Σ|X _(S)|²   (64″)

ΔH _(LS) +ΔH _(RS) =ΣX _(M)*·2E _(S) /Σ|X _(M)|²   (65″)

ΔH _(LS) −ΔH _(RS) =ΣX _(S)*·2E _(S) /Σ|X _(S)|²   (66″)

[0262] From the equations (63″) to (66″),

ΔH _(LM) =ΣX _(M)*·2E _(M) /Σ|X _(M)|² +Σ _(S)*·2E _(M) /Σ|X _(S)|²  (67)

ΔH _(RM) =ΣX _(M)*·2E _(M) /Σ|X _(M)|² −Σ _(S)*·2E _(M) /Σ|X _(S)|²  (68)

ΔH _(LS) =ΣX _(M)*·2E _(S) /Σ|X _(M)|² +Σ _(S)*·2E _(S) /Σ|X _(S)|²  (69)

ΔH _(RS) =ΣX _(M)*·2E _(S) /Σ|X _(M)|² −Σ _(S)*·2E _(S) /Σ|X _(S)|²  (70)

[0263] are respectively derived.

[0264] Using the estimated errors ΔH_(LM), ΔH_(RM), ΔH_(LS) and ΔH_(RS)derived from the equations (67) to (70), the filter characteristics ofthe filter means 240-1, 240-2, 240-3 and 240-4 are updated per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging). For example, assuming that impulse responsesh_(LM), h_(RM), h_(LS) and h_(RS) after K-th updating are given ash_(LM)(k), h_(RM)(k), h_(LS)(k) and h_(RS)(k), using impulse responsesΔh_(LM), Δh_(RM), Δh_(LS) and Δh_(RS) corresponding to the derivedestimated errors ΔH_(LM), ΔH_(RM), ΔH_(LS) and ΔH_(RS),

h _(LM)(k+1)=h _(LM)(k)+αΔh _(LM)   (71)

h _(RM)(k+1)=h _(RM)(k)+αΔh _(RM)   (72)

h _(LS)(k+1)=h _(LS)(k)+αΔh _(LS)   (73)

h _(RS)(k+1)=h _(RS)(k)+αΔh _(RS)   (74).

[0265] Using these updating equations, (k+1)th impulse responsesh_(LM)(k+1), h_(RM)(k+1), h_(LS)(k+1) and h_(RS)(k+1) are derived andset to the filter means 240-1, 240-2, 240-3 and 240-4, respectively,which is repeated per suitably determined prescribed time period (e.g.time period of performing ensemble averaging).

[0266]FIG. 17 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2. The same symbols are used with respect tothose portions common to the foregoing structure of FIG. 16. Left/righttwo-channel stereo signals x_(L) and x_(R) transmitted from the spot onthe counterpart side and inputted into line input ends LI(L) and LI(R)are outputted from sound output ends SO(L) and SO(R) as they are (i.e.not through sum/difference signal producing means 252), and reproducedat loudspeakers SP(L) and SP(R), respectively. The sum/difference signalproducing means 252 performs addition of such stereo signals x_(L) andx_(R) using an adder 254 so as to produce a sum signal x_(M)(=x_(L)+x_(R)), while performs subtraction thereof using a subtracter256 so as to produce a difference signal x_(S) {=x_(L)−x_(R) (or it mayalso be x_(R)−x_(L))}.

[0267] Transfer function calculating means 258 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) produced by the sum/difference signal producingmeans 252 and signals e_(M) and e_(S) outputted from subtracters 248 and250 and, based on this cross-spectrum calculation, performs setting andupdating of filter characteristics of the filter means 240-1 to 240-4.Operations thereof are the same as those described with respect to thestructure of FIG. 16. The signals e_(M) and e_(S) outputted from thesubtracters 248 and 250 are inputted into stereo audio signaldemodulating means 282. The stereo audio signal demodulating means 282performs addition of the signals e_(M) and e_(S) using an adder 284, andfurther, gives thereto a coefficient 1/2 using a coefficient multiplier286 to recover a left-channel signal e_(L), while performs subtractionof the signals e_(M) and e_(S) using a subtracter 288, and further,gives thereto a coefficient 1/2 using a coefficient multiplier 290 torecover a right-channel signal e_(R). The recovered signals e_(L) ande_(R) are respectively outputted from line output ends LO(L) and LO(R)and transmitted toward the spot on the counterpart side. Operations ofthe other portions are the same as those described with respect to thestructure of FIG. 16.

[0268]FIG. 18 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2. The same symbols are used with respect tothose portions common to the foregoing structure of FIG. 16 or 17. A sumsignal x_(M) (=x_(L)+x_(R)) and a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))} transmitted from the spot on thecounterpart side and inputted into line input ends LI(L) and LI(R) areinputted into stereo audio signal demodulating means 262. The stereoaudio signal demodulating means 262 performs addition of the sum anddifference signals x_(M) and x_(S) using an adder 264, and further,gives thereto a coefficient 1/2 using a coefficient multiplier 266 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 268, andfurther, gives thereto a coefficient 1/2 using a coefficient multiplier270 to recover the original signal x_(R). The recovered signals x_(L)and x_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively. Sum/differencesignal producing means 252 performs addition of such stereo signalsx_(L) and x_(R) using an adder 254 so as to produce a sum signal x_(M)(=x_(L)+x_(R)), while performs subtraction thereof using a subtracter256 so as to produce a difference signal x_(S) {=x_(L)−x_(R) (or it mayalso be x_(R)−x_(L))}.

[0269] Transfer function calculating means 258 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) produced by the sum/difference signal producingmeans 252 and signals e_(M) and e_(S) outputted from subtracters 248 and250 and, based on this cross-spectrum calculation, performs setting andupdating of filter characteristics of filter means 240-1 to 240-4.Operations thereof are the same as those described with respect to thestructure of FIG. 16 or 17. The signals e_(M) and e_(S) outputted fromthe subtracters 248 and 250 are respectively outputted from line outputends LO(L) and LO(R) and transmitted toward the spot on the counterpartside. Operations of the other portions are the same as those describedwith respect to the structure of FIG. 16 or 17.

[0270]FIG. 19 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2. The same symbols are used with respect tothose portions common to the foregoing structure of FIG. 16, 17 or 18.Left/right two-channel stereo signals x_(L) and x_(R) transmitted fromthe spot on the counterpart side and inputted into line input ends LI(L)and LI(R) are inputted into sum/difference signal producing means 252.The sum/difference signal producing means 252 performs addition of thestereo signals x_(L) and x_(R) using an adder 254 so as to produce a sumsignal x_(M) (=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 256 so as to produce a difference signal x_(S) {=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}. The produced sum signal x_(M) anddifference signal x_(S) are inputted into stereo audio signaldemodulating means 262. The stereo audio signal demodulating means 262performs addition of the sum and difference signals x_(M) and x_(S)using an adder 264, and further, gives thereto a coefficient 1/2 using acoefficient multiplier 266 to recover the original signal x_(L), whileperforms subtraction of the sum and difference signals x_(m) and x_(S)using a subtracter 268, and further, gives thereto a coefficient 1/2using a coefficient multiplier 270 to recover the original signal x_(R).The recovered signals x_(L) and x_(R) are outputted from sound outputends SO(L) and SO(R) and reproduced at loudspeakers SP(L) and SP(R),respectively.

[0271] Transfer function calculating means 258 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) produced by the sum/difference signal producingmeans 252 and signals e_(M) and e_(S) outputted from subtracters 248 and250 and, based on this cross-spectrum calculation, performs setting andupdating of filter characteristics of the filter means 240-1 to 240-4.Operations thereof are the same as those described with respect to thestructure of FIG. 16, 17 or 18. The signals e_(M) and e_(S) outputtedfrom the subtracters 248 and 250 are inputted into stereo audio signaldemodulating means 282. The stereo audio signal demodulating means 282performs addition of the signals e_(M) and e_(S) using an adder 284, andfurther, gives thereto a coefficient 1/2 using a coefficient multiplier286 to recover a left-channel signal e_(L), while performs subtractionof the signals e_(M) and e_(S) using a subtracter 288, and further,gives thereto a coefficient 1/2 using a coefficient multiplier 290 torecover a right-channel signal e_(R). The recovered signals e_(L) ande_(R) are respectively outputted from line output ends LO(L) and LO(R)and transmitted toward the spot on the counterpart side. Operations ofthe other portions are the same as those described with respect to thestructure of FIG. 16, 17 or 18.

[0272]FIG. 20 shows another structural example in the stereo echocanceller 16, 24. A sum signal x_(M) (=x_(L)+x_(R)) and a differencesignal x_(S) {=x_(L)−x_(R) (or it may also be x_(R)−x_(L))} transmittedfrom the spot on the counterpart side and inputted into line input endsLI(L) and LI(R) are inputted into stereo audio signal demodulating means362. The stereo audio signal demodulating means 362 performs addition ofthe sum and difference signals x_(M) and x_(S) using an adder 364, andfurther, gives thereto a coefficient 1/2 using a coefficient multiplier366 to recover the original signal x_(L), while performs subtraction ofthe sum and difference signals x_(M) and x_(S) using a subtracter 368,and further, gives thereto a coefficient 1/2 using a coefficientmultiplier 370 to recover the original signal x_(R). The recoveredsignals x_(L) and x_(R) are outputted from sound output ends SO(L) andSO(R) and reproduced at loudspeakers SP(L) and SP(R), respectively.

[0273] Collected audio signals y_(L) and y_(R) of microphones MC(L) andMC(R) are inputted into sum/difference signal producing means 372. Thesum/difference signal producing means 372 implements addition of themicrophone collected audio signals y_(L) and y_(R) using an adder 373 soas to produce a sum signal y_(M), while implements subtraction thereofusing a subtracter 375 so as to produce a difference signal y_(S).

[0274] Filer means 340-1 to 340-4 are formed by, for example, FIRfilters. These filter means 340-1 to 340-4 are each set with an impulseresponse corresponding to a composite transfer function in the form ofcombination of transfer functions H_(LL), H_(LR), H_(RL) and H_(RR) offour audio transfer systems between the loudspeakers SP(L) and SP(R) andmicrophones MC(L) and MC(R), respectively, and perform, using suchimpulse responses, a convolution calculation of the sum signal x_(M) andthe difference signal x_(S) inputted from the line input ends LI(L) andLI(R), thereby producing echo cancel signals EC1 to EC4, respectively.

[0275] An adder 344 performs a calculation of EC1+EC3. An adder 346performs a calculation of EC2+EC4. A subtracter 348 subtracts an echocancel signal EC1+EC3 from the sum signal y_(M), thereby to perform echocancellation. A subtracter 350 subtracts an echo cancel signal EC2+EC4from the difference signal y_(S), thereby to perform echo cancellation.Signals e_(M) and e_(S) outputted from the subtracters 348 and 350 areoutputted from line output ends LO(L) and LO(R), respectively, andtransmitted toward the spot on the counterpart side.

[0276] Transfer function calculating means 358 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) inputted from the line input ends LI(L) andLI(R) and the signals e_(M) and e_(S) outputted from the subtracters 348and 350 and, based on this cross-spectrum calculation, performs settingand updating of filter characteristics (impulse responses) of the filtermeans 340-1 to 340-4. Specifically, upon starting the system, the filtercharacteristics of the filter means 340-1 to 340-4 are not set, i.e.coefficients are all set to zero, so that the echo cancel signals EC1 toEC4 are zero, and thus the sum signal y_(M) and the difference signaly_(S) outputted from the sum/difference signal producing means 372, asthey are, are outputted from the subtracters 348 and 350. Therefore, atthis time, the transfer function calculating means 358 performs thecross-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) inputted from the line input ends LI(L) andLI(R) and the sum signal e_(M) and the difference signal e_(S) outputtedfrom the subtracters 348 and 350 and, based on this cross-spectrumcalculation, derives a plurality of composite transfer functions each inthe form of combination of transfer functions H_(LL), H_(LR), H_(RL) andH_(RR) of four audio transfer systems between the loudspeakers SP(L) andSP(R) and the microphones MC(L) and MC(R), respectively, and implementsinitial setting of the filter characteristics of the filter means 340-1to 340-4 to values corresponding to such composite transfer functions.After the initial setting, since the echo cancel signals are produced bythe filter means 340-1 to 340-4, the echo cancel error signals e_(M) ande_(S) corresponding to difference signals between the sum signal y_(M)and the difference signal y_(S) outputted from the sum/difference signalproducing means 372 and the echo cancel signals EC1 to EC4 are outputtedfrom the subtracters 348 and 350. Therefore, at this time, the transferfunction calculating means 358 performs the cross-spectrum calculationbetween the sum signal x_(M) and the difference signal x_(S) inputtedfrom the line input ends LI(L) and LI(R) and the echo cancel errorsignals e_(M) and e_(S) outputted from the subtracters 348 and 350 and,based on this cross-spectrum calculation, derives estimated errors ofthe foregoing composite transfer functions, respectively, and updatesthe filter characteristics of the filter means 340-1 to 340-4 to valuesthat cancel such estimated errors, respectively. By repeating thisupdating operation per prescribed time period, the echo cancel error canbe converged to a minimum value. Further, even if the transfer functionschange due to movement of the microphone positions or the like, the echocancel error can be converged to a minimum value by sequentiallyupdating the filter characteristics of the filter means 340-1 to 340-4depending thereon.

[0277] Correlation detecting means 360 detects a correlation between thesum signal x_(M) and the difference signal x_(S) based on a correlationvalue calculation or the like, and stops updating of the foregoingfilter characteristics when the correlation value is no less than aprescribed value. When the correlation value becomes lower than theprescribed value, updating of the foregoing filter characteristics isrestarted.

[0278] Herein, the filter characteristics (impulse responses) that areset to the filter means 340-1 to 340-4 by the transfer functioncalculating means 358 will be described. In the transfer functioncalculating means 358, the following calculation is performed.

[0279] (In Case of Fixed Type Operation)

[0280] The sum and difference signals y_(M) and y_(S) of the outputsignals y_(L) and y_(R) of the microphones MC(L) and MC(R) become$\begin{matrix}\begin{matrix}{Y_{M} = {Y_{L} + Y_{R}}} \\{= {\left( {{X_{L} \cdot H_{LL}} + {X_{R} \cdot H_{RL}}} \right) + \left( {{X_{L} \cdot H_{LR}} + {X_{R} \cdot H_{RR}}} \right)}} \\{= {{X_{L}\left( {H_{LL} + H_{LR}} \right)} + {X_{R}\left( {H_{RL} + H_{RR}} \right)}}}\end{matrix} & (71) \\\begin{matrix}{Y_{S} = {Y_{L} - Y_{R}}} \\{= {\left( {{X_{L} \cdot H_{LL}} + {X_{R} \cdot H_{RL}}} \right) - \left( {{X_{L} \cdot H_{LR}} + {X_{R} \cdot H_{RR}}} \right)}} \\{= {{X_{L}\left( {H_{LL} - H_{LR}} \right)} + {{X_{R}\left( {H_{RL} - H_{RR}} \right)}.}}}\end{matrix} & (72)\end{matrix}$

X _(L)=(X _(M) +X _(S))/2

X _(R)=(X _(M) −X _(S))/2

[0281] hence, the equations (71) and (72) respectively become$\begin{matrix}\begin{matrix}{Y_{M} = {{\left( {X_{M} + X_{S}} \right) \cdot {\left( {H_{LL} + H_{LR}} \right)/2}} +}} \\{{\left( {X_{M} - X_{S}} \right) \cdot {\left( {H_{RL} + H_{RR}} \right)/2}}} \\{= {{{X_{M}\left( {H_{LL} + H_{LR} + H_{RL} + H_{RR}} \right)}/2} +}} \\{{{X_{S}\left( {H_{LL} + H_{LR} - H_{RL} - H_{RR}} \right)}/2}}\end{matrix} & \left( {71'} \right) \\\begin{matrix}{Y_{S} = {{\left( {X_{M} + X_{S}} \right) \cdot {\left( {H_{LL} - H_{LR}} \right)/2}} +}} \\{{\left( {X_{M} - X_{S}} \right) \cdot {\left( {H_{RL} - H_{RR}} \right)/2}}} \\{= {{{X_{M}\left( {H_{LL} - H_{LR} + H_{RL} - H_{RR}} \right)}/2} +}} \\{{{X_{S}\left( {H_{LL} - H_{LR} - H_{RL} + H_{RR}} \right)}/2}}\end{matrix} & \left( {72'} \right)\end{matrix}$

[0282] When the composite transfer functions are given as

H _(MM)=(H _(LL) +H _(LR) +H _(RL) +H _(RR))/2

H _(SM)=(H _(LL) +H _(LR) −H _(RL) −H _(RR))/2

H _(MS)=(H _(LL) −H _(LR) +H _(RL) −H _(RR))/2

H _(SS)=(H _(LL) −H _(LR) −H _(RL) +H _(RR))/2

[0283] the equations (71′) and (72′) respectively become

Y _(M) =X _(M) ·H _(MM) +X _(S) ·H _(SM)   (71″)

Y _(S) =X _(M) ·H _(MS) +X _(S) /H _(SS)   (72″).

[0284] When both sides of the equations (71″) and (72″) are multipliedby complex conjugates X_(M)* and X_(S)* of X_(M) and X_(S) andensemble-averaged,

ΣX _(M) *·Y _(M) =ΣX _(M) *·X _(M) ·H _(MM) +ΣX _(M) *·X _(S) ·H _(SM)  (73)

ΣX _(S) *·Y _(M) =ΣX _(S) *·X _(M) ·H _(MM) +ΣX _(S) *·X _(S) ·H _(SM)  (74)

ΣX _(M) *·Y _(S) =ΣX _(M) *·X _(M) ·H _(MS) +ΣX _(M) *·X _(S) ·H _(SS)  (75)

ΣX _(S) *·Y _(S) =ΣX _(M) *·X _(M) ·H _(MS) +ΣX _(S) *·X _(S) ·H _(SS)  (76)

[0285] are respectively obtained.

[0286] In the equations (73) to (76), since X_(M) and X_(S) areapproximately uncorrelated with each other, such a term havingX_(M)*·X_(S) or X_(S)*·X_(M) becomes approximately zero whenensemble-averaged. Further,

X _(M) *·X _(M) =|X _(M)|²

X _(S) *·X _(S) =|X _(S)|²

[0287] hence, the equations (73) to (76) respectively become

ΣX _(M) *·Y _(M) =Σ|X _(M)|² ·H _(MM)   (73′)

ΣX _(S) *·Y _(M) =Σ|X _(S)|² ·H _(SM)   (74′)

ΣX _(M) *·Y _(S) =Σ|X _(M)|² ·H _(MS)   (75′)

ΣX _(S) *·Y _(S) =Σ|X _(S)|² ·H _(SS)   (76′).

[0288] From the equations (73′) to (76′),

H _(MM) =ΣX _(M) *·Y _(M) /Σ|X _(M)|²   (77)

H _(SM) =ΣX _(S) *·Y _(M) /Σ|X _(S)|²   (78)

H _(MS) =ΣX _(M) *·Y _(S) /Σ|X _(M)|²   (79)

H _(SS) =ΣX _(S) *·Y _(S) /Σ|X _(S)|²   (80)

[0289] are respectively derived.

[0290] Impulse responses h_(MM), h_(SM), h_(MS) and h_(SS) obtained byapplying the inverse Fourier transformation to these derived compositetransfer functions H_(MM), H_(SM), H_(MS) and H_(SS) are the filtercharacteristics to be set to the filter means 340-1, 340-2, 340-3 and340-4, respectively. Therefore, the transfer function calculating means358 derives the respective composite transfer functions H_(MM), H_(SM),H_(MS) and H_(SS) from the equations (77) to (80) based on the sumsignal x_(M) and the difference signal x_(S) inputted into the lineinput ends LI(L) and LI(R) and the sum signal y_(M) and the differencesignal y_(S) outputted from the sum/difference signal producing means372, derives the impulse responses h_(MM), h_(SM), h_(MS) and h_(SS) byapplying the inverse Fourier transformation to those derived compositetransfer functions, sets the derived impulse responses to the filtermeans 340-1, 340-2, 340-3 and 340-4, respectively, and further, updatesthe impulse responses by repeating this calculation per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging).

[0291] (In Case of Adaptive Type Operation)

[0292] Assuming that the filter characteristics set to the filter means340-1, 340-2, 340-3 and 340-4 are given as H{circumflex over ( )}_(MM),H{circumflex over ( )}_(SM), H{circumflex over ( )}_(MS) andH{circumflex over ( )}_(SS) (h{circumflex over ( )}_(MM), h{circumflexover ( )}_(SM), h{circumflex over ( )}_(MS) and h{circumflex over( )}_(SS) when expressed in terms of the impulse responses), the signalse_(M) and e_(S) outputted from the subtracters 348 and 350 become$\begin{matrix}\begin{matrix}{E_{M} = \left\{ {{\left( {X_{M} + X_{S}} \right) \cdot {\left( {H_{LL} + H_{LR}} \right)/2}} + {\left( {X_{M} - X_{S}} \right) \cdot}} \right.} \\{\left. {\left( {H_{RL} + H_{RR}} \right)/2} \right\} - \left( {{X_{M} \cdot H_{MM}^{\bigwedge}} + {H_{S} \cdot H_{SM}^{\bigwedge}}} \right)} \\{= {{X_{M}\left\lbrack {\left\{ {\left( {H_{LL} + H_{LR} + H_{RL} + H_{RR}} \right)/2} \right\} - H_{MM}^{\bigwedge}} \right\rbrack} +}} \\{{X_{S}\left\lbrack {\left\{ {\left( {H_{{LL}\quad} + H_{LR} - H_{RL} - H_{RR}} \right)/2} \right\} - H_{SM}^{\bigwedge}} \right\rbrack}}\end{matrix} & (81) \\\begin{matrix}{E_{S} = \left\{ {{\left( {X_{M} + X_{S}} \right) \cdot {\left( {H_{LL} - H_{LR}} \right)/2}} + {\left( {X_{M} - X_{S}} \right) \cdot}} \right.} \\{\left. {\left( {H_{RL} - H_{RR}} \right)/2} \right\} - \left( {{X_{M} \cdot H_{MS}^{\bigwedge}} + {X_{S} \cdot H_{SS}^{\bigwedge}}} \right)} \\{= {{X_{M}\left\lbrack {\left\{ {\left( {H_{LL} - H_{LR} + H_{RL} - H_{RR}} \right)/2} \right\} - H_{MS}^{\bigwedge}} \right\rbrack} +}} \\{{X_{S}\left\lbrack {\left\{ {\left( {H_{{LL}\quad} - H_{LR} - H_{RL} + H_{RR}} \right)/2} \right\} - H_{SS}^{\bigwedge}} \right\rbrack}}\end{matrix} & (82)\end{matrix}$

[0293] When the composite transfer functions are given as

H _(MM)=(H _(LL) +H _(LR) +H _(RL) +H _(RR))/2

H _(SM)=(H _(LL) +H _(LR) −H _(RL) −H _(RR))/2

H _(MS)=(H _(LL) −H _(LR) +H _(RL) −H _(RR))/2

H _(SS)=(H _(LL) −H _(LR) −H _(RL) +H _(RR))/2

[0294] the equations (81) and (82) respectively become

E _(M) =X _(M)(H _(MM) −H{circumflex over ( )} _(MM))+X _(S)(H _(SM)−H{circumflex over ( )} _(SM))   (81′)

E _(S) =X _(M)(H _(MS) −H{circumflex over ( )} _(MS))+X _(S)(H _(SS)−H{circumflex over ( )} _(SS))   (82′).

[0295] When

ΔH _(MM) =H _(MM) −H{circumflex over ( )} _(MM)

ΔH _(SM) =H _(SM) −H{circumflex over ( )} _(SM)

ΔH _(MS) =H _(MS) −H{circumflex over ( )} _(MS)

ΔH _(SS) =H _(SS) −H{circumflex over ( )} _(SS)

[0296] are given, the equations (81′) and (82′) respectively become

E _(M) =X _(M) ·ΔH _(MM) +X _(S) ·ΔH _(SM)   (81″)

E _(S) =X _(M) ·ΔH _(MS) +X _(S) ·ΔH _(SS)   (82″).

[0297] When both sides of the equations (81″) and (82″) are multipliedby complex conjugates X_(M)* and X_(S)* of X_(M) and X_(S) andensemble-averaged,

ΣX _(M) *·E _(M) =ΣX _(M) *·X _(M) ·ΔH _(MM) +ΣX _(M) *·X _(S) ·ΔH _(SM)  (83)

ΣX _(S) *·E _(M) =ΣX _(S) *·X _(M) ·ΔH _(MM) +ΣX _(S) *·X _(S) ·ΔH _(SM)  (84)

ΣX _(M) *·E _(S) =ΣX _(M) *·X _(M) ·ΔH _(MS) +ΣX _(M) *·X _(S) ·ΔH _(SS)  (85)

ΣX _(S) *·E _(S) =ΣX _(S) *·X _(M) ·ΔH _(MS) +ΣX _(S) *·X _(S) ·ΔH _(SS)  (86)

[0298] are respectively obtained.

[0299] In the equations (83) to (86), since X_(M) and X_(S) areapproximately uncorrelated with each other, such a term havingX_(M)*·X_(S) or X_(S)*·X_(M) becomes approximately zero whenensemble-averaged. Further,

X _(M) *·X _(M) =|X _(M)|²

X _(S) *·X _(S) =|X _(S)|²

[0300] hence, the equations (83) to (86) respectively become

ΣX _(M) *·E _(M) =Σ|X _(M)|² ·ΔH _(MM)   (83′)

ΣX _(S) *·E _(M) =Σ|X _(S)|² ·ΔH _(SM)   (84′)

ΣX _(M) *·E _(S) =Σ|X _(M)|² ·ΔH _(MS)   (85′)

ΣX _(S) *·E _(S) =Σ|X _(S)|² ·ΔH _(SS)   (86′).

[0301] From the equations (83′) to (86′),

ΔH _(MM) =ΣX _(M) *·E _(M) /Σ|X _(M)|²   (87)

ΔH _(SM) =ΣX _(S) *·E _(M) /Σ|X _(S)|²   (88)

ΔH _(MS) =ΣX _(M) *·E _(S) /Σ|X _(M)|²   (89)

ΔH _(SS) =ΣX _(S) *·E _(S) /Σ|X _(S)|²   (90)

[0302] are respectively derived.

[0303] Using the estimated errors ΔH_(MM), ΔH_(SM), ΔH_(MS) and ΔH_(SS)derived from the equations (87) to (90), the filter characteristics ofthe filter means 340-1, 340-2, 340-3 and 340-4 are updated per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging). For example, assuming that impulse responsesh_(MM), h_(SM), h_(MS) and h_(SS) after K-th updating are given ash_(MM)(k), h_(SM)(k), h_(MS)(k) and h_(SS)(k), using impulse responsesΔh_(MM), Δh_(MS), Δh_(MS) and Δh_(SS) corresponding to the derivedestimated errors ΔH_(MM), ΔH_(SM), ΔH_(MS) and ΔH_(SS),

h _(MM)(k+1)=h _(MM)(k)+αΔh _(MM)   (91)

h _(SM)(k+1)=h _(SM)(k)+αΔh _(SM)   (92)

h _(MS)(k+1)=h _(MS)(k)+αΔh _(MS)   (93)

h _(SS)(k+1)=h _(SS)(k)+αΔh _(SS)   (94).

[0304] Using these updating equations, (k+1)th impulse responsesh_(MM)(k+1), h_(SM)(k+1), h_(MS)(k+1) and h_(SS)(k+1) are derived andset to the filter means 340-1, 340-2, 340-3 and 340-4, respectively,which is repeated per suitably determined prescribed time period (e.g.time period of performing ensemble averaging).

[0305]FIG. 21 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2. The same symbols are used with respect tothose portions common to the foregoing structure of FIG. 20. Left/righttwo-channel stereo signals x_(L) and x_(R) transmitted from the spot onthe counterpart side and inputted into line input ends LI(L) and LI(R)are outputted from sound output ends SO(L) and SO(R) as they are (i.e.not through sum/difference signal producing means 352), and reproducedat loudspeakers SP(L) and SP(R), respectively. The sum/difference signalproducing means 352 performs addition of such stereo signals x_(L) andx_(R) using an adder 354 so as to produce a sum signal x_(M)(=x_(L)+x_(R)), while performs subtraction thereof using a subtracter356 so as to produce a difference signal x_(S){=x_(L)−x_(R) (or it mayalso be x_(R)−x_(L))}.

[0306] Transfer function calculating means 358 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) produced by the sum/difference signal producingmeans 352 and signals e_(M) and e_(S) outputted from subtracters 348 and350 and, based on this cross-spectrum calculation, performs setting andupdating of filter characteristics of the filter means 340-1 to 340-4.Operations thereof are the same as those described with respect to thestructure of FIG. 20. The signals e_(M) and e_(S) outputted from thesubtracters 348 and 350 are inputted into stereo audio signaldemodulating means 382. The stereo audio signal demodulating means 382performs addition of the signals e_(M) and e_(S) using an adder 384, andfurther, gives thereto a coefficient 1/2 using a coefficient multiplier386 to recover a left-channel signal e_(L), while performs subtractionof the signals e_(M) and e_(S) using a subtracter 388, and further,gives thereto a coefficient 1/2 using a coefficient multiplier 390 torecover a right-channel signal e_(R). The recovered signals e_(L) ande_(R) are respectively outputted from line output ends LO(L) and LO(R)and transmitted toward the spot on the counterpart side. Operations ofthe other portions are the same as those described with respect to thestructure of FIG. 20.

[0307]FIG. 22 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2. The same symbols are used with respect tothose portions common to the foregoing structure of FIG. 20 or 21. A sumsignal x_(M) (=x_(L)+x_(R)) and a difference signal x_(S) {=x_(L)−x_(R)(or it may also be x_(R)−x_(L))} transmitted from the spot on thecounterpart side and inputted into line input ends LI(L) and LI(R) areinputted into stereo audio signal demodulating means 362. The stereoaudio signal demodulating means 362 performs addition of the sum anddifference signals x_(M) and x_(S) using an adder 364, and further,gives thereto a coefficient 1/2 using a coefficient multiplier 366 torecover the original signal x_(L), while performs subtraction of the sumand difference signals x_(M) and x_(S) using a subtracter 368, andfurther, gives thereto a coefficient 1/2 using a coefficient multiplier370 to recover the original signal x_(R). The recovered signals x_(L)and x_(R) are outputted from sound output ends SO(L) and SO(R) andreproduced at loudspeakers SP(L) and SP(R), respectively. Sum/differencesignal producing means 352 performs addition of such stereo signalsx_(L) and x_(R) using an adder 354 so as to produce a sum signal x_(M)(=x_(L)+x_(R)), while performs subtraction thereof using a subtracter356 so as to produce a difference signal x_(S) {=x_(L)−x_(R) (or it mayalso be x_(R)−x_(L))}.

[0308] Transfer function calculating means 358 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) produced by the sum/difference signal producingmeans 352 and signals e_(M) and e_(S) outputted from subtracters 348 and350 and, based on this cross-spectrum calculation, performs setting andupdating of filter characteristics of filter means 340-1 to 340-4.Operations thereof are the same as those described with respect to thestructure of FIG. 20 or 21. The signals e_(M) and e_(S) outputted fromthe subtracters 348 and 350 are respectively outputted from line outputends LO(L) and LO(R) and transmitted toward the spot on the counterpartside. Operations of the other portions are the same as those describedwith respect to the structure of FIG. 20 or 21.

[0309]FIG. 23 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2. The same symbols are used with respect tothose portions common to the foregoing structure of FIG. 20, 21 or 22.Left/right two-channel stereo signals x_(L) and x_(R) transmitted fromthe spot on the counterpart side and inputted into line input ends LI(L)and LI(R) are inputted into sum/difference signal producing means 352.The sum/difference signal producing means 352 performs addition of thestereo signals x_(L) and x_(R) using an adder 354 so as to produce a sumsignal x_(M) (=x_(L)+x_(R)), while performs subtraction thereof using asubtracter 356 so as to produce a difference signal x_(S){=x_(L)−x_(R)(or it may also be x_(R)−x_(L))}. The produced sum signal x_(M) anddifference signal x_(S) are inputted into stereo audio signaldemodulating means 362. The stereo audio signal demodulating means 362performs addition of the sum and difference signals x_(M) and x_(S)using an adder 364, and further, gives thereto a coefficient 1/2 using acoefficient multiplier 366 to recover the original signal x_(L), whileperforms subtraction of the sum and difference signals x_(M) and x_(s)using a subtracter 368, and further, gives thereto a coefficient 1/2using a coefficient multiplier 370 to recover the original signal x_(R).The recovered signals x_(L) and x_(R) are outputted from sound outputends SO(L) and SO(R) and reproduced at loudspeakers SP(L) and SP(R),respectively.

[0310] Transfer function calculating means 358 implements across-spectrum calculation between the sum signal x_(M) and thedifference signal x_(S) produced by the sum/difference signal producingmeans 352 and signals e_(M) and e_(S) outputted from subtracters 348 and350 and, based on this cross-spectrum calculation, performs setting andupdating of filter characteristics of the filter means 340-1 to 340-4.Operations thereof are the same as those described with respect to thestructure of FIG. 20, 21 or 22. The signals e_(M) and e_(S) outputtedfrom the subtracters 348 and 350 are inputted into stereo audio signaldemodulating means 382. The stereo audio signal demodulating means 382performs addition of the signals e_(M) and e_(S) using an adder 384, andfurther, gives thereto a coefficient 1/2 using a coefficient multiplier386 to recover a left-channel signal e_(L), while performs subtractionof the signals e_(M) and e_(S) using a subtracter 388, and further,gives thereto a coefficient 1/2 using a coefficient multiplier 390 torecover a right-channel signal e_(R). The recovered signals e_(L) ande_(R) are respectively outputted from line output ends LO(L) and LO(R)and transmitted toward the spot on the counterpart side. Operations ofthe other portions are the same as those described with respect to thestructure of FIG. 20, 21 or 22.

[0311] This invention can take various structures in addition to thestructures as shown in the foregoing embodiments. For example, part ofthe structures of FIGS. 1, 9 to 11 can be changed like FIG. 24.Specifically, sum/difference signal producing means 402 is disposedbetween the adders 44 and 46 and the subtracters 48 and 50, so as toperform addition of the echo cancel signals EC1+EC3 and EC2+EC4 using anadder 404 to produce an echo cancel signal (EC1+EC3)+(EC2+EC4), whileperform subtraction thereof using a subtracter 406 to produce(EC1+EC3)−(EC2+EC4). Further, sum/difference signal producing means 408is disposed between the sound input ends SI(L) and SI(R) and thesubtracters 48 and 50, so as to perform addition of the collected audiosignals y_(L) and y_(R) of the microphones MC(L) and MC(R) using anadder 410 to produce a sum signal y_(M), while perform subtractionthereof using a subtracter 412 to produce a difference signal y_(S). Thesubtracter 48 subtracts the echo cancel signal (EC1+EC3)+(EC2+EC4) fromthe sum signal y_(M) to implement echo cancellation. The subtracter 50subtracts the echo cancel signal (EC1+EC3)−(EC2+EC4) from the differencesignal y_(S) to implement echo cancellation. The signals e_(M) and e_(S)outputted from the subtracters 48 and 50 are inputted into stereo audiosignal demodulating means 414. The stereo audio signal demodulatingmeans 414 performs addition of the signals e_(M) and e_(S) using anadder 416, and further, gives thereto a coefficient 1/2 using acoefficient multiplier 418 to recover a left-channel signal e_(L), whileperforms subtraction of the signals e_(M) and e_(S) using a subtracter420, and further, gives thereto a coefficient 1/2 using a coefficientmultiplier 422 to recover a right-channel signal e_(R). The otherportions are the same as those described with respect to the structuresof FIGS. 1, 9 to 11.

[0312] On the other hand, part of the structures of FIGS. 12 to 15 canbe changed like FIG. 25. Specifically, sum/difference signal producingmeans 432 is disposed between the adders 144 and 146 and the subtracters148 and 150, so as to perform addition of the echo cancel signalsEC1+EC3 and EC2+EC4 using an adder 434 to produce an echo cancel signal(EC1+EC3)+(EC2+EC4), while perform subtraction thereof using asubtracter 436 to produce (EC1+EC3)−(EC2+EC4). Further, sum/differencesignal producing means 438 is disposed between the sound input endsSI(L) and SI(R) and the subtracters 148 and 150, so as to performaddition of the collected audio signals y_(L) and y_(R) of themicrophones MC(L) and MC(R) using an adder 440 to produce a sum signaly_(M), while perform subtraction thereof using a subtracter 442 toproduce a difference signal y_(S). The subtracter 148 subtracts the echocancel signal (EC1+EC3)+(EC2+EC4) from the sum signal y_(M) to implementecho cancellation. The subtracter 150 subtracts the echo cancel signal(EC1+EC3)−(EC2+EC4) from the difference signal y_(S) to implement echocancellation. The signals e_(M) and e_(S) outputted from the subtracters148 and 150 are inputted into stereo audio signal demodulating means444. The stereo audio signal demodulating means 444 performs addition ofthe signals e_(M) and e_(S) using an adder 446, and further, givesthereto a coefficient 1/2 using a coefficient multiplier 448 to recovera left-channel signal e_(L), while performs subtraction of the signalse_(M) and e_(S) using a subtracter 450, and further, gives thereto acoefficient 1/2 using a coefficient multiplier 452 to recover aright-channel signal e_(R). The other portions are the same as thosedescribed with respect to the structures of FIGS. 12 to 15.

[0313] On the other hand, part of the structures of FIGS. 16 to 19 canbe changed like FIG. 26. Specifically, sum/difference signal producingmeans 462 is disposed between the adders 244 and 246 and the subtracters248 and 250, so as to perform addition of the echo cancel signalsEC1+EC3 and EC2+EC4 using an adder 464 to produce an echo cancel signal(EC1+EC3)+(EC2+EC4), while perform subtraction thereof using asubtracter 466 to produce (EC1+EC3)−(EC2+EC4). The sum/difference signalproducing means 272 of FIGS. 16 to 19 is not required. The subtracter248 subtracts the echo cancel signal (EC1+EC3)+(EC2+EC4) from thecollected audio signal y_(L) of the microphone MC(L) to implement echocancellation. The subtracter 250 subtracts the echo cancel signal(EC1+EC3)−(EC2+EC4) from the collected audio signal y_(R) of themicrophone MC(R) to implement echo cancellation. Sum/difference signalproducing means 468 is disposed on the output side of the subtracters248 and 250, so as to perform addition of the output signals e_(L) ande_(R) of the subtracters 248 and 250 using an adder 470 to produce a sumsignal e_(M), while perform subtraction thereof using a subtracter 472to produce a difference signal e_(S). The other portions are the same asthose described with respect to the structures of FIGS. 16 to 19.

[0314] On the other hand, part of the structures of FIGS. 20 and 21 canbe changed like FIG. 27. Specifically, sum/difference signal producingmeans 482 is disposed between the adders 344 and 346 and the subtracters348 and 350, so as to perform addition of the echo cancel signalsEC1+EC3 and EC2+EC4 using an adder 484 to produce an echo cancel signal(EC1+EC3)+(EC2+EC4), while perform subtraction thereof using asubtracter 486 to produce (EC1+EC3)−(EC2+EC4). The sum/difference signalproducing means 372 of FIGS. 20 to 23 is not required. The subtracter348 subtracts the echo cancel signal (EC1+EC3)+(EC2+EC4) from thecollected audio signal y_(L) of the microphone MC(L) to implement echocancellation. The subtracter 350 subtracts the echo cancel signal(EC1+EC3)−(EC2+EC4) from the collected audio signal y_(R) of themicrophone MC(R) to implement echo cancellation. Sum/difference signalproducing means 488 is disposed on the output side of the subtracters348 and 350, so as to perform addition of the output signals e_(L) ande_(R) of the subtracters 348 and 350 using an adder 490 to produce a sumsignal e_(M), while perform subtraction thereof using a subtracter 492to produce a difference signal e_(S). The other portions are the same asthose described with respect to the structures of FIGS. 20 to 23.

[0315]FIG. 28 shows another structural example in the stereo echocanceller 16, 24 of FIG. 2. The same symbols are used with respect tothose portions common to the foregoing structure of FIG. 1. Left/righttwo-channel stereo signals x_(L) and x_(R) transmitted from the spot onthe counterpart side and inputted into line input ends LI(L) and LI(R)are outputted from sound output ends SO(L) and SO(R) as they are (i.e.not through an orthogonalizing filter 500), and reproduced atloudspeakers SP(L) and SP(R), respectively.

[0316] Filter means 40-1 is set with an impulse response correspondingto a transfer function between the loudspeaker SP(L) and a microphoneMC(L), filter means 40-2 is set with an impulse response correspondingto a transfer function between the loudspeaker SP(L) and a microphoneMC(R), filter means 40-3 is set with an impulse response correspondingto a transfer function between the loudspeaker SP(R) and the microphoneMC(L), and filter means 40-4 is set with an impulse responsecorresponding to a transfer function between the loudspeaker SP(R) andthe microphone MC(R).

[0317] The orthogonalizing filter 500 performs a principal componentanalysis with respect to the input stereo signals x_(L) and x_(R) perprescribed time period, and converts such input stereo signals x_(L) andx_(R) into two signals that are orthogonal to each other. Transferfunction calculating means 502 implements a cross-spectrum calculationbetween the mutually orthogonal two signals produced at theorthogonalizing filter 500 and signals e_(L) and e_(R) outputted fromsubtracters 48 and 50 and, based on this cross-spectrum calculation,sets filter characteristics (impulse responses) of the filter means 40-1to 40-4. Specifically, upon starting the system, the filtercharacteristics of the filter means 40-1 to 40-4 are not set, i.e.coefficients are all set to zero, so that echo cancel signals EC1 to EC4are zero, and thus collected audio signals of the microphones MC(L) andMC(R) themselves are outputted from the subtracters 48 and 50.Therefore, at this time, the transfer function calculating means 502performs the cross-spectrum calculation between the mutually orthogonaltwo signals produced at the orthogonalizing filter 500 and the collectedaudio signals e_(L) and e_(R) of the microphones MC(L) and MC(R)outputted from the subtracters 48 and 50 and, based on thiscross-spectrum calculation, derives transfer functions of four audiotransfer systems between the loudspeakers SP(L) and SP(R) and themicrophones MC(L) and MC(R), respectively, and implements initialsetting of the filter characteristics of the filter means 40-1 to 40-4to values corresponding to such transfer functions. After the initialsetting, since the echo cancel signals are produced by the filter means40-1 to 40-4, the echo cancel error signals e_(L) and e_(R)corresponding to difference signals between the collected audio signalsof the microphones MC(L) and MC(R) and the echo cancel signals EC1 toEC4 are outputted from the subtracters 48 and 50. Therefore, at thistime, the transfer function calculating means 502 performs thecross-spectrum calculation between the mutually orthogonal two signalsproduced at the orthogonalizing filter 500 and the echo cancel errorsignals e_(L) and e_(R) outputted from the subtracters 48 and 50 and,based on this cross-spectrum calculation, derives estimated errors ofthe transfer functions of the four audio transfer systems between theloudspeakers SP(L) and SP(R) and the microphones MC(L) and MC(R),respectively, and updates the filter characteristics of the filter means40-1 to 40-4 to values that cancel the estimated errors, respectively.By repeating this updating operation per prescribed time period, theecho cancel error can be converged to a minimum value. Further, even ifthe transfer functions change due to movement of the microphonepositions or the like, the echo cancel error can be converged to aminimum value by sequentially updating the filter characteristics of thefilter means 40-1 to 40-4 depending thereon.

[0318] According to the known technique based on comparison between theleft/right two-channel stereo signals x_(L) and x_(R) inputted into theline input ends LI(L) and LI(R) and the signals e_(L) and e_(R) to beoutputted from line output ends LO(L) and LO(R), double talk detectingmeans 504 detects the double talk where sounds other than thosereproduced by the loudspeakers SP(L) and SP(R) are inputted into themicrophones MC(L) and MC(R). The transfer function calculating means 502makes relatively longer an update period of the filter characteristicsof the filter means 40-1 to 40-4 while the double talk is detected,whereas makes relatively shorter the update period of the filtercharacteristics while the double talk is not detected. This makes itpossible to fully converge estimated errors when the double talk exists,and further, quicken convergence of estimated errors when there is nodouble talk.

[0319] An orthogonalization process of the orthogonalizing filter 500will be described. The orthogonalization process is implemented perprescribed time period of the input stereo signals. Herein, theorthogonalization process is carried out per frame (e.g. 512 samples) asshown in FIG. 29 (however, overlap processing may be implemented asshown in FIG. 49 which will be described later). A vector having as itselements a sample group of one frame of the left-channel input signalx_(L) inputted into the orthogonalizing filter 500, and a vector havingas its elements a sample group of one frame of the right-channel inputsignal x_(R) are given as

{overscore (x _(L))}=(x _(L1) , x _(L2) , x _(L3) , . . . , x _(Ln))

{overscore (x _(R))}=(x _(R1) , x _(R2) , x _(R3) , . . . , x _(Rn))  [Equation 1]

[0320] Since x_(L) and x_(R) are the stereo signals, they mutually havea correlation. The orthogonalization process is carried out byperforming a principal component analysis, using x_(L) and x_(R) as twovariables, with respect to a sample group composed of combinations ofsuch two variables per frame so as to derive eigenvectors of the firstprincipal component and the second principal component that are mutuallyorthogonal, and projecting the respective samples composed ofcombinations of such two variables onto the derived eigenvectors of thefirst principal component and the second principal component,respectively.

[0321] Concrete contents of calculation of the orthogonalization processwill be described. Now, assuming that an observation matrix B is givenas $\begin{matrix}{B = \begin{pmatrix}{x_{L1},x_{L2},x_{L3},\ldots \quad,x_{L\quad n}} \\{x_{R1},x_{R2},x_{R3},\ldots \quad,x_{R\quad n}}\end{pmatrix}} & \left\lbrack {{Equation}\quad 2} \right\rbrack\end{matrix}$

[0322] then, a covariance matrix S of B becomes $\begin{matrix}\begin{matrix}{S = {\frac{1}{n - 1}{BB}^{T}\quad \left( {B^{T}\quad {is}\quad a\quad {transposed}\quad {matrix}\quad {of}\quad B} \right)}} \\{= {\frac{1}{n - 1}\begin{pmatrix}{x_{L1},x_{L2},x_{L3},\ldots \quad,x_{L\quad n}} \\{x_{R1},x_{R2},x_{R3},\ldots \quad,x_{R\quad n}}\end{pmatrix}\begin{pmatrix}x_{L1} & x_{R1} \\x_{L2} & x_{R2} \\. & . \\. & . \\x_{L\quad n} & x_{Rn}\end{pmatrix}}} \\{= {\frac{1}{n - 1}\begin{pmatrix}{\sum\limits_{k = 1}^{n}\quad x_{Lk}^{2}} & {\sum\limits_{k = 1}^{n}\quad {x_{Lk}x_{Rk}}} \\{\sum\limits_{k = 1}^{n}\quad {x_{Lk}x_{Rk}}} & {\sum\limits_{k = 1}^{n}\quad x_{Rk}^{2}}\end{pmatrix}}} \\{= \begin{pmatrix}S_{11} & S_{12} \\S_{21} & S_{22}\end{pmatrix}}\end{matrix} & \left\lbrack {{Equation}\quad 3} \right\rbrack\end{matrix}$

[0323] (S₁₁ is variance of x_(L), S₂₂ is variance of x_(R), andS₁₂(=S₂₁) is covariance of x_(L) and x_(R))

[0324] Hence, from $\begin{matrix}{\begin{pmatrix}{S_{11} - \lambda} & S_{12} \\S_{21} & {S_{22} - \lambda}\end{pmatrix} = 0} & \left\lbrack {{Equation}\quad 4} \right\rbrack\end{matrix}$

 (S ₁₁−λ)(S ₂₂−λ)−S ₁₂ S ₂₁=0   [Equation 5]

[0325] is solved, thus $\begin{matrix}{\lambda = \frac{\begin{matrix}{S_{11} + {S_{22} \pm}} \\\sqrt{\left( {S_{11} + S_{22}} \right)^{2} - {4\left( {{S_{11}S_{22}} - S_{12}^{2}} \right)}}\end{matrix}}{2}} & \left\lbrack {{Equation}\quad 6} \right\rbrack\end{matrix}$

[0326] so that two solutions for eigenvalues λ are derived.

[0327] Assuming that one of the two eigenvalues having greater variance(eigenvalue of the first principal component) is λ₁, an eigenvectorU_(max) corresponding to the eigenvalue λ₁ is $\begin{matrix}\begin{pmatrix}u_{11} \\u_{12}\end{pmatrix} & \left\lbrack {{Equation}\quad 8} \right\rbrack\end{matrix}$

[0328] which establishes $\begin{matrix}{{\begin{pmatrix}S_{11} & S_{12} \\S_{21} & S_{22}\end{pmatrix}\begin{pmatrix}u_{11} \\u_{12}\end{pmatrix}} = {\lambda_{1}\begin{pmatrix}u_{11} \\u_{12}\end{pmatrix}}} & \left\lbrack {{Equation}\quad 7} \right\rbrack\end{matrix}$

[0329] where u₁₁ ²+u₁₂ ²=1

[0330] By solving u₁₁ and u₁₂, $\begin{matrix}{{u_{11} = {\pm \frac{S_{12}}{\sqrt{S_{12}^{2} - \left( {\lambda_{1} - S_{11}} \right)^{2}}}}}{u_{12} = {\pm \frac{\lambda_{1} - S_{11}}{\sqrt{S_{12}^{2} - \left( {\lambda_{1} - S_{11}} \right)^{2}}}}}\left( {{double}\quad {signs}\quad {in}\quad {same}\quad {order}} \right)} & \left\lbrack {{Equation}\quad 9} \right\rbrack\end{matrix}$

[0331] are derived. Regardless of the sign of u₁₁ and u₁₂ being plus orminus, the axis represented by the first principal component is thesame.

[0332] On the other hand, assuming that one of the two eigenvalueshaving smaller variance (eigenvalue of the second principal component)is λ₂, an eigenvector U_(min) corresponding to the eigenvalue X₂ is$\begin{matrix}\begin{pmatrix}u_{21} \\u_{22}\end{pmatrix} & \left\lbrack {{Equation}\quad 11} \right\rbrack\end{matrix}$

[0333] which establishes $\begin{matrix}{{\begin{pmatrix}S_{11} & S_{12} \\S_{21} & S_{22}\end{pmatrix}\begin{pmatrix}u_{21} \\u_{22}\end{pmatrix}} = {\lambda_{2}\begin{pmatrix}u_{21} \\u_{22}\end{pmatrix}}} & \left\lbrack {{Equation}\quad 10} \right\rbrack\end{matrix}$

[0334] where u₂₁ ²+u₂₂ ²=1

[0335] By solving u₂₁ and u₂₂, $\begin{matrix}\begin{matrix}{u_{21} = {\pm \frac{S_{12}}{\sqrt{S_{12}^{2} - \left( {\lambda_{2} - S_{11}} \right)^{2}}}}} \\{u_{22} = {\pm \frac{\lambda_{2} - S_{11}}{\sqrt{S_{12}^{2} - \left( {\lambda_{2} - S_{11}} \right)^{2}}}}} \\\left( {{double}\quad {signs}\quad {in}\quad {same}\quad {order}} \right)\end{matrix} & \left\lbrack {{Equation}\quad 12} \right\rbrack\end{matrix}$

[0336] are derived. Regardless of the sign of u₂₁ and u₂₂ being plus orminus, the axis represented by the second principal component is thesame.

[0337] From the foregoing, the eigenvector U_(max) of the firstprincipal component and the eigenvector U_(min) of the second principalcomponent are derived as follows. $\begin{matrix}{{\overset{\rightarrow}{U_{\max}} = \begin{pmatrix}u_{11} \\u_{12}\end{pmatrix}},{\overset{\rightarrow}{U_{\min}} = \begin{pmatrix}u_{21} \\u_{22}\end{pmatrix}}} & \left\lbrack {{Equation}\quad 13} \right\rbrack\end{matrix}$

[0338] With respect to the eigenvectors U_(max) and U_(min) obtainedfrom the covariance matrix, it is not possible to predict in whichquadrant it appears due to its nature. If the quadrant in which theeigenvector U_(max) appears changes per frame, U_(max)'s mutually cancelthemselves upon ensemble-averaging U_(max)'s per block later. If thequadrant in which the eigenvector U_(min) appears changes per frame,U_(min)'s mutually cancel themselves upon ensemble-averaging U_(min)'sper block later. Therefore, a conversion operation is performed forfixing the quadrants in which the eigenvectors U_(max) and U_(min)appear. For example, when fixing the eigenvector U_(max) to the firstquadrant and the eigenvector U_(min) to the fourth quadrant, it can berealized by the following conversion operation.

[0339] Of the eigenvectors U_(max) and U_(min),

[0340] one existing in the first quadrant (positive, positive) or thethird quadrant (negative, negative) is given as U_(max)′, and

[0341] one existing in the second quadrant (negative, positive) or thefourth quadrant (positive, negative) is given as U_(min)′. Then,conversion is executed such that

[0342] when U_(max)′ exists in the first quadrant, U_(max)=U_(max)′

[0343] when U_(max)′ exists in the third quadrant, U_(max)=U_(max)′

[0344] when U_(min)′ exists in the second quadrant, U_(min)=U_(min)′

[0345] when U_(min)′ exists in the fourth quadrant, U_(min)=U_(min)′.

[0346] Through such a conversion operation, the quadrants in which theeigenvectors U_(max) and U_(min) appear can be fixed.

[0347] Onto the eigenvector U_(max) of the first principal component andthe eigenvector U_(min) of the second principal component that areobtained as described above, a column vector of the observation matrix Bgiven as $\begin{matrix}{\overset{\rightarrow}{b} = \begin{pmatrix}x_{L\quad n} \\x_{R\quad n}\end{pmatrix}} & \left\lbrack {{Equation}\quad 14} \right\rbrack\end{matrix}$

[0348] is projected. A value of an output signal x_(max) obtained byprojecting the observation matrix B onto the eigenvector U_(max) isderived as

x_(max)={right arrow over (b)}{right arrow over (U_(max))} (represents inner product)   [Equation 15]

[0349] Further, a value of an output signal x_(min) obtained byprojecting the observation matrix B onto the eigenvector U_(min) isderived as

x_(min)={right arrow over (b)}{right arrow over (U_(min))} (represents inner product)   [Equation 16]

[0350] The transfer function calculation process of the transferfunction calculating means 502 will be described.

[0351] (In Case of Fixed Type Operation)

[0352] A filter characteristic of the orthogonalizing filter 500 isgiven as U. The filter characteristic U is a characteristic thatprojects the input signals x_(L) and x_(R) onto the mutuallyuncorrelated signals x_(max) and x_(min) based on the principalcomponent analysis, and the following relationship is established.$\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\quad 17} \right\rbrack \\{\left. \begin{matrix}{\begin{pmatrix}\overset{\rightarrow}{x_{\max}} \\\overset{\rightarrow}{x_{\min}}\end{pmatrix} = {U\begin{pmatrix}\overset{\rightarrow}{x_{L}} \\\overset{\rightarrow}{x_{R}}\end{pmatrix}}} \\{U = \begin{pmatrix}u_{11} & u_{12} \\u_{21} & u_{22}\end{pmatrix}}\end{matrix} \right\} \quad}\end{matrix} & (95)\end{matrix}$

[0353] An inverse characteristic of the filter characteristic U is givenas V. The inverse filter characteristic V is a characteristic thatrestores the signals x_(max) and x_(min) to the original signals x_(L)and x_(R), and the following relationship is established.$\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\quad 18} \right\rbrack \\{\left. \begin{matrix}{{UV} = {{VU} = {I\quad \left( {I\quad {is}\quad a\quad {unit}\quad {matrix}} \right)}}} \\{V = \begin{pmatrix}v_{11} & v_{12} \\v_{21} & v_{22}\end{pmatrix}}\end{matrix} \right\} \quad}\end{matrix} & (96)\end{matrix}$

[0354] Output signals y_(L) and y_(R) of the microphones MC(L) and MC(R)are expressed as

Y _(L) =H _(LL) ·X _(L) +H _(RL) ·X _(R)   (97)

Y _(R) =H _(LR) ·X _(L) +H _(RR) ·X _(R)   (98).

[0355] From the equations (95) and (96), $\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\quad 19} \right\rbrack \\{\begin{pmatrix}x_{L} \\x_{R}\end{pmatrix} = {\begin{pmatrix}v_{11} & v_{12} \\v_{21} & v_{22}\end{pmatrix}\begin{pmatrix}x_{\max} \\x_{\min}\end{pmatrix}}}\end{matrix} & (99)\end{matrix}$

[0356] hence, assuming that frequency-axis expressions of x_(max) andx_(min) are respectively given as X_(max) and X_(min), the equations(97) and (98) respectively become

Y _(L) =H _(LL)(v ₁₁ ·X _(max) +v ₁₂ ·X _(min))+H _(RL)(v ₂₁ ·X _(max)+v ₂₂ ·X _(min))   (97′)

Y _(R) =H _(LR)(v ₁₁ ·X _(max) +v ₁₂ ·X _(min))+H _(RR)(v ₂₁ ·X _(max)+v ₂₂ ·X _(min))   (98′).

[0357] When both sides of the equation (97′) are multiplied by complexconjugates X*_(max) and X*_(min) of X_(max) and X_(min) (i.e. derivingcross spectra) and ensemble-averaged, because X_(max) and X_(min) aremutually orthogonal, expected values of X*_(max)·X_(min) andX*_(min)·X_(max) become zero, respectively, so that the following twoequations are obtained (note: E[ ] represents the ensemble average).

E[X* _(max) ·Y _(L) ]=E[X* _(max) ·H _(LL) ·v ₁₁ ·X _(max) +X* _(max) ·H_(RL) ·v ₂₁ ·X _(max)]  (100)

E[X* _(min) ·Y _(L) ]=E[X* _(min) ·H _(LL) ·v ₁₂ ·X _(min) +X* _(min) ·H_(RL) ·v ₂₂ ·X _(min)]  (101)

[0358] Similarly, when both sides of the equation (98′) are multipliedby complex conjugates X*_(max) and X*_(min) of X_(max) and X_(min) andensemble-averaged, the following two equations are obtained.

E[X* _(max) ·Y _(R) ]=E[X* _(max) ·H _(LR) ·v ₁₁ ·X _(max) +X* _(max) ·H_(RR) ·v ₂₁ ·X _(max)]  (102)

E[X* _(min) ·Y _(R) ]=E[X* _(min) ·H _(LR) ·v ₁₂ ·X _(min) +X* _(min) +H_(RR) ·v ₂₂ ·X _(min)]  (103)

[0359] Here, if a change of the eigenvalues λ is small in a time periodof performing ensemble averaging, $\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\quad 20} \right\rbrack \\\left. \begin{matrix}{U = {{\begin{pmatrix}u_{11} & u_{12} \\u_{21} & u_{22}\end{pmatrix} \approx {E\lbrack U\rbrack}} = \begin{pmatrix}{E\left\lbrack u_{11} \right\rbrack} & {E\left\lbrack u_{12} \right\rbrack} \\{E\left\lbrack u_{21} \right\rbrack} & {E\left\lbrack u_{22} \right\rbrack}\end{pmatrix}}} \\{V = {{\begin{pmatrix}v_{11} & v_{12} \\v_{21} & v_{22}\end{pmatrix} \approx {E\lbrack V\rbrack}} = \begin{pmatrix}{E\left\lbrack v_{11} \right\rbrack} & {E\left\lbrack v_{12} \right\rbrack} \\{E\left\lbrack v_{21} \right\rbrack} & {E\left\lbrack v_{22} \right\rbrack}\end{pmatrix}}}\end{matrix} \right\}\end{matrix} & (104)\end{matrix}$

[0360] is established, so that the equations (100) to (103) arerewritten as the following equations (100′) to (103′).

E[X* _(max) ·Y _(L) ]≈E[|X _(max)|² ]·H _(LL) ·E[v ₁₁ ]+E[|X _(max)|²]·H _(RL) ·E[v ₂₁]  (100′)

E[X* _(min) ·Y _(L) ]≈E[|X _(min)|² ]·H _(LL) ·E[v ₁₂ ]+E[|X _(min)|²]·H _(RL) ·E[v ₂₂]  (101′)

E[X* _(max) ·Y _(R) ]≈E[|X _(max)|² ]·H _(LR) ·E[v ₁₁ ]+E[|X _(max)|²]·H _(RR) ·E[v ₂₁]  (102′)

E[X* _(min) ·Y _(R) ]≈E[|X _(min)|²]·H_(LR) ·E[v₁₂]+E[|X_(min)|²]·H_(RR) ·E[v ₂₂]  (103′)

[0361] When both sides of the equations (100′) and (102′) are divided byE[|X_(max)|²] and both sides of the equations (101′) and (103′) aredivided by E[|X_(min)|²], respectively,

E[X* _(max) ·Y _(L) ]/E[|X _(max)|² ]≈H _(LL) ·E[v ₁₁ ]+H _(RL) ·E[v ₂₁]

E[X* _(min) ·Y _(L) ]/E[|X _(min)|² ]≈H _(LL) ·E[v ₁₂ ]+H _(RL) ·E[v ₂₂]

E[X* _(max) ·Y _(R) ]/E[|X _(max)|² ]≈H _(LR) ·E[v ₁₁ ]+H _(RR) ·E[v ₂₁]

E[X* _(min) ·Y _(R) ]/E[|X _(min)|² ]≈H _(LR) ·E[v ₁₂ ]+H _(RR) ·E[v ₂₂]

[0362] hence $\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\quad 21} \right\rbrack \\{\begin{pmatrix}\frac{E\left\lbrack {X_{\max}^{*} \cdot Y_{L}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot Y_{L}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack} \\\frac{E\left\lbrack {X_{\max}^{*} \cdot Y_{R}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot Y_{R}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack}\end{pmatrix} \approx {\begin{pmatrix}H_{LL} & H_{RL} \\H_{LR} & H_{RR}\end{pmatrix}\begin{pmatrix}{E\left\lbrack v_{11} \right\rbrack} & {E\left\lbrack v_{12} \right\rbrack} \\{E\left\lbrack v_{21} \right\rbrack} & {E\left\lbrack v_{22} \right\rbrack}\end{pmatrix}}}\end{matrix} & (105)\end{matrix}$

[0363] is obtained. From E[U]·E[V]≈I, the equation (105) is rewritten as$\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\quad 22} \right\rbrack \\{\begin{pmatrix}H_{LL} & H_{RL} \\H_{LR} & H_{RR}\end{pmatrix} \approx {\begin{pmatrix}\frac{E\left\lbrack {X_{\max}^{*} \cdot Y_{L}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot Y_{L}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack} \\\frac{E\left\lbrack {X_{\max}^{*} \cdot Y_{R}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot Y_{R}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack}\end{pmatrix}\begin{pmatrix}{E\left\lbrack u_{11} \right\rbrack} & {E\left\lbrack u_{12} \right\rbrack} \\{E\left\lbrack u_{21} \right\rbrack} & {E\left\lbrack u_{22} \right\rbrack}\end{pmatrix}}}\end{matrix} & (106)\end{matrix}$

[0364] Impulse responses h_(LL), h_(RL), h_(LR) and h_(RR) obtained byapplying the inverse Fourier transformation to the transfer functionsH_(LL), H_(RL), H_(LR) and H_(RR) derived from the equation (106) arethe filter characteristics to be set to the filter means 40-1, 40-2,40-3 and 40-4, respectively. Therefore, the transfer functioncalculating means 502 derives the respective transfer functions H_(LL),H_(RL), H_(LR) and H_(RR) based on the signals x_(max) and x_(min)outputted from the orthogonalizing filter 500, the filter characteristicU of the orthogonalizing filter 500 and the output signals y_(L) andy_(R) of the microphones MC(L) and MC(R), derives the impulse responsesh_(LL), h_(RL), h_(LR) and h_(RR) by applying the inverse Fouriertransformation to those derived transfer functions, sets the derivedimpulse responses to the filter means 40-1, 40-2, 40-3 and 40-4,respectively, and further, updates the impulse responses by repeatingthis calculation per suitably determined prescribed time period (e.g.time period of performing ensemble averaging).

[0365] (In Case of Adaptive Type Operation)

[0366] Assuming that the filter characteristics set to the filter means40-1, 40-2, 40-3 and 40-4 are given as H{circumflex over ( )}_(LL),H{circumflex over ( )}_(RL), H{circumflex over ( )}_(LR) andH{circumflex over ( )}_(RR) (h{circumflex over ( )}_(LL), h{circumflexover ( )}_(RL), h{circumflex over ( )}_(LR) and h{circumflex over( )}_(RR) when expressed in terms of the impulse responses), the signalse_(L) and e_(R) outputted from the subtracters 48 and 50 of FIG. 28 areexpressed as

E _(L)=(H _(LL) ·X _(L) +H _(RL) ·X _(R))−(H{circumflex over ( )} _(LL)·X _(L) +H{circumflex over ( )} _(RL) ·X _(R))   (107)

E _(R)=(H _(LR) ·X _(L) +H _(RR) ·X _(R))−(H{circumflex over ( )} _(LR)·X _(L) +H{circumflex over ( )} _(RR) ·X _(R))   (108).

[0367] From the foregoing equation (99), the equations (107) and (108)respectively become

E _(L)=(H _(LL) −H{circumflex over ( )} _(LL))(v ₁₁ ·X _(max) +v ₁₂ ·X_(min))+(H _(RL) −H{circumflex over ( )} _(RL))(v ₂₁ ·X _(max) +v ₂₂ ·X_(min))   (107′)

E _(R)=(H _(LR) −H{circumflex over ( )} _(LR))(v ₁₁ ·X _(max) +v ₁₂ ·X_(min))+(H _(RR) −H{circumflex over ( )} _(RR))(v₂₁ ·X _(max) +V ₂₂ ·X_(min))   (108′).

[0368] When the estimated errors of the transfer functions are given as

ΔH _(LL) =H _(LL) −H{circumflex over ( )} _(LL)

ΔH _(RL) =H _(RL) −H{circumflex over ( )} _(RL)

ΔH _(LR) =H _(LR) −H{circumflex over ( )} _(LR)

ΔH _(RR) =H _(RR) −H{circumflex over ( )} _(RR)

[0369] the equations (107′) and (108′) respectively become

E _(L) =ΔH _(LL)(v ₁₁ ·X _(max) +v ₁₂ ·X _(min))+ΔH _(RL)(v ₂₁ ·X _(max)+v ₂₂ ·X _(min))   (107″)

E _(R) =ΔH _(LR)(v ₁₁ ·X _(max) +v ₁₂ ·X _(min))+ΔH _(RR)(v ₂₁·X_(max)+v ₂₂ ·X _(min))   (108″).

[0370] When both sides of the equation (107″) are multiplied by complexconjugates X*_(max) and X*_(min) of X_(max) and X_(min) (i.e. derivingcross spectra) and ensemble-averaged, because X_(max) and X_(min) aremutually orthogonal, expected values of X*_(max)·X_(min) andX*_(min)·X_(max) become zero, respectively, so that the following twoequations are obtained (note: E[ ] represents the ensemble average).

E[X* _(max) ·E _(L) ]=E[X* _(max) ·ΔH _(LL) ·v ₁₁ ·X _(max) +X* _(max)·ΔH _(RL) ·v ₂₁ ·X _(max)]  (109)

E[X* _(min) ·E _(L) ]=E[X* _(min) ·ΔH _(LL) ·v ₁₂ ·X _(min) +X* _(min)·ΔH _(RL) ·v ₂₂ ·X _(min)]  (110)

[0371] Similarly, when both sides of the equation (108″) are multipliedby complex conjugates X*_(max) and X*_(min) of X_(max) and X_(min) andensemble-averaged, the following two equations are obtained.

E[X* _(max) ·E _(R) ]=E[X* _(max) ·ΔH _(LR) ·v ₁₁ ·X _(max) +X* _(max)·ΔH _(RR) ·v ₂₁ ·X _(max) ]  (111)

E[X* _(min) ·E _(R) ]=E[X* _(min) ·ΔH _(LR) ·v ₁₂ ·X _(min) +X* _(min)·ΔH _(RR) ·v ₂₂ ·X _(min)]  (112)

[0372] Here, if a change of the eigenvalues λ is small in a time periodof performing ensemble averaging, the foregoing equation (104) isestablished so that the equations (109) to (112) are rewritten as thefollowing equations (109′) to (112′).

E[X* _(max) ·E _(L) ]≈E[|X _(max)|² ]·ΔH _(LL) ·E[v ₁₁ ]+E[|X _(max)|²]·ΔH _(RL) ·E[v ₂₁]  (109′)

E[X* _(min) ·E _(L) ]≈E[|X _(min)|² ]·ΔH _(LL) ·E[v ₁₂ ]+E[|X _(min)|²]·ΔH _(RL) ·E[v ₂₂]  (110′)

E[X* _(max) ·E _(R) ]≈E[|X _(max)|² ]·ΔH _(LR) ·E[v ₁₁ ]+E[|X _(max)|²]·ΔH _(RR) ·E[v ₂₁]  (111′)

E[X* _(min) ·E _(R) ]≈E[|X _(min)|² ]·ΔH _(LR) ·E[v ₁₂ ]+E[|X _(min)|²]·ΔH _(RR) ·E[v ₂₂]  (112′)

[0373] When both sides of the equations (109′) and (111′) are divided byE[|X_(max)|²] and both sides of the equations (110′) and (112′) aredivided by E[|X_(min)|²], respectively,

E[X* _(max) ·E _(L) ]/E[|X _(max)|² ]≈ΔH _(LL) ·E[v ₁₁ ]+ΔH _(RL) ·E[v_(21])

E[X* _(min) ·E _(L) ]/E[|X _(min)|² ]≈ΔH _(LL) ·E[v ₁₂ ]+ΔH _(RL) ·E[v_(22])

E[X* _(max) ·E _(R) ]/E[|X _(max)|² ]≈ΔH _(LR) ·E[v ₁₁ ]+ΔH _(RR) ·E[v_(21])

E[X* _(min) ·E _(R) ]/E[|X _(min)|² ]≈ΔH _(LR) ·E[v ₁₁ ]+ΔH _(RR) ·E[v_(22])

[0374] hence $\begin{matrix}\begin{matrix}\left\lbrack {{Equation}\quad 23} \right\rbrack \\{\begin{pmatrix}\frac{E\left\lbrack {X_{\max}^{*} \cdot E_{L}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot E_{L}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack} \\\frac{E\left\lbrack {X_{\max}^{*} \cdot E_{R}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot E_{R}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack}\end{pmatrix} \approx {\begin{pmatrix}{\Delta \quad H_{LL}} & {\Delta \quad H_{RL}} \\{\Delta \quad H_{LR}} & {\Delta \quad H_{RR}}\end{pmatrix}\begin{pmatrix}{E\left\lbrack v_{11} \right\rbrack} & {E\left\lbrack v_{12} \right\rbrack} \\{E\left\lbrack v_{21} \right\rbrack} & {E\left\lbrack v_{22} \right\rbrack}\end{pmatrix}}}\end{matrix} & (113)\end{matrix}$

[0375] is obtained. From E[U]·E[V]≈I, the equation (113) is rewritten as$\begin{matrix}{\left\lbrack {{Equation}\quad 24} \right\rbrack {\begin{pmatrix}{\Delta \quad H_{LL}} & {\Delta \quad H_{RL}} \\{\Delta \quad H_{LR}} & {\Delta \quad H_{RR}}\end{pmatrix} \approx {\begin{pmatrix}\frac{E\left\lbrack {X_{\max}^{*} \cdot E_{L}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot E_{L}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack} \\\frac{E\left\lbrack {X_{\max}^{*} \cdot E_{R}} \right\rbrack}{E\left\lbrack {X_{\max}}^{2} \right\rbrack} & \frac{E\left\lbrack {X_{\min}^{*} \cdot E_{R}} \right\rbrack}{E\left\lbrack {X_{\min}}^{2} \right\rbrack}\end{pmatrix}\begin{pmatrix}{E\left\lbrack u_{11} \right\rbrack} & {E\left\lbrack u_{12} \right\rbrack} \\{E\left\lbrack u_{21} \right\rbrack} & {E\left\lbrack u_{22} \right\rbrack}\end{pmatrix}}}} & (114)\end{matrix}$

[0376] Using the estimated errors ΔH_(LL), ΔH_(RL), ΔH_(LR) and ΔH_(RR)derived from the equation (114), the filter characteristics of thefilter means 40-1, 40-2, 40-3 and 40-4 are updated per suitablydetermined prescribed time period (e.g. time period of performingensemble averaging). For example, assuming that impulse responsesh_(LL), h_(RL), h_(LR) and h_(RR) after K-th updating are given ash_(LL)(k), h_(RL)(k), h_(LR)(k) and h_(RR)(k), using impulse responsesΔh_(LL), ΔH_(RL), Δh_(LR) and Δh_(RR) corresponding to the derivedestimated errors ΔH_(LL), ΔH_(RL), ΔH_(LR) and ΔH_(RR),

h _(LL)(k+1)=h _(LL)(k)+αΔh _(LL)

h _(RL)(k+1)=h _(RL)(k)+αΔh _(RL)

h _(LR)(k+1)=h _(LR)(k)+αΔh _(LR)

h _(RR)(k+1)=h _(RR)(k)+αΔh _(RR).

[0377] Using these updating equations, (k+1)th impulse responsesh_(LL)(k+1), h_(RL)(k+1), h_(LR)(k+1) and h_(RR)(k+1) are derived andset to the filter means 40-1, 40-2, 40-3 and 40-4, respectively, whichis repeated per suitably determined prescribed time period (e.g. timeperiod of performing ensemble averaging).

[0378]FIG. 30 shows functional blocks of the orthogonalizing filter 500of FIG. 28. The input stereo signals x_(L) and x_(R) are inputted frominput ends 506 and 508, respectively. Covariance matrix calculatingmeans 510 derives a covariance matrix S of the input stereo signalsx_(L) and x_(R) per frame. Based on the derived covariance matrix S,eigenvector calculating means 512 derives coefficients u₁₁, u₁₂, u₂, andu₂₂ of eigenvectors U_(max) and U_(min) of the first and secondprincipal components per frame. The derived coefficients u₁₁, u₂₁, u₁₂and u₂₂ are set to coefficient multipliers 514, 516, 518 and 520,respectively. The coefficient multipliers 514 and 516 give thecoefficients u₁₁ and u₂₁ to the input signal x_(L) to derive x_(L)·u₁₁and x_(L)·u₂₁, respectively. The coefficient multipliers 518 and 520give the coefficients u₁₂ and u₂₂ to the input signal x_(R) to derivex_(R)·U₁₂ and x_(R)·U₂₂, respectively. An adder 522 derivesx_(L)·U₁₁+x_(R)·U₁₂ as a signal x_(max) obtained by projecting the inputstereo signals x_(L) and x_(R) onto the eigenvector U_(max). An adder524 derives x_(L)·u₂₁+x_(R)·u₂₂ as a signal x_(min) obtained byprojecting the input stereo signals x_(L) and x_(R) onto the eigenvectorU_(min). The signals X_(max) and x_(min) are respectively outputted fromoutput ends 526 and 528, while the coefficients u₁₁, u₂₁, u₁₂ and u₂₂ ofthe eigenvectors are outputted from an output end 530.

[0379]FIG. 31 shows functional blocks of the transfer functioncalculating means 502 of FIG. 28. The signals x_(max) and x_(min) arerespectively inputted from input ends 532 and 534, while thecoefficients u₁₁, u₂₁, u₁₂ and u₂₂ of the eigenvectors are inputted froman input end 536. FFT means 538 applies the fast Fourier transformationto the signal x_(max). Complex conjugate calculating means 540calculates a complex conjugate X*_(max) of X_(max). Power spectrumcalculating means 542 calculates X_(max)·X*_(max)=|X_(max)|². Ensembleaveraging means 544 calculates E[|X_(max)|²]. FFT means 546 applies thefast Fourier transformation to the signal x_(min). Complex conjugatecalculating means 548 calculates a complex conjugate X*_(min) ofX_(min). Power spectrum calculating means 550 calculatesX_(min)·X*_(min)=X_(min)|². Ensemble averaging means 552 calculatesE[|X_(min)|²].

[0380] An input end 554 is inputted with an output signal y_(L) of themicrophone MC(L) (or output signal E_(L) of the subtracter 48). FFTmeans 558 applies the fast Fourier transformation to the signal y_(L)(or signal E_(L)). Cross spectrum calculating means 560 derivesX*_(max)·Y_(L) (or X*_(max)·E_(L)) , and ensemble averaging means 562derives E[X*_(max)·Y_(L)] (or E[X*_(max)·E_(L)]). Cross spectrumcalculating means 564 derives X*_(min)·Y_(L) (or X*_(min)·E_(L)), andensemble averaging means 566 derives E[X*_(min)·Y_(L)] (orE[X*_(min)·E_(L)]).

[0381] An input end 568 is inputted with an output signal y_(R) of themicrophone MC(R) (or output signal E_(R) of the subtracter 50). FFTmeans 570 applies the fast Fourier transformation to the signal y_(R)(or signal E_(R)). Cross spectrum calculating means 572 derivesX*_(max)·Y_(R) (or X*_(max)·E_(R)) , and ensemble averaging means 574derives E[X*_(max)·Y_(R)] (or E[X*_(max)·E_(R)]). Cross spectrumcalculating means 576 derives X*_(min)·Y_(R) (or X*_(min)·E_(R)), andensemble averaging means 578 derives E[X*_(min)·Y_(R)] (orE[X*_(min)·E_(R)]).

[0382] Composite transfer function calculating means 580 derives thefirst term on the right side of the equation (106) {or the equation(114)} based on E[|X_(max)|²], E[|X_(min)|²], E[X*_(max)·Y_(L)] (orE[X*_(max)·E_(L)]), E[X*_(min)·Y_(L)] (or E[X*_(min)·E_(L)]),E[X*_(max)·Y_(R)] (or E[X*_(max)·E_(R)]) , and E[X*_(min)·Y_(R)] (orE[X*_(min)·E_(R)]) which are derived as described above. Averaging means582 averages the coefficients u₁₁, u₂₁, u₁₂ and u₂₂ of the eigenvectorsindividually to derive E [v₁₁], E [v₁₂], E [v₂₁] and E [v₂₂].

[0383] Based on the outputs of the composite transfer functioncalculating means 580 and the averaging means 582, transfer functioncalculating means 584 performs a calculation of the right side of theequation (106) {or the equation (114)} to derive individual transferfunctions H_(LL), H_(RL), H_(LR) and H_(RR) (or their estimated errorsΔH_(LL), ΔH_(RL), ΔH_(LR) and ΔH_(RR)). Inverse FFT means 586 appliesthe inverse fast Fourier transformation to the derived transferfunctions H_(LL), H_(RL), H_(LR) and H_(RR) (or their estimated errorsΔH_(LL), ΔH_(RL), ΔH_(LR) and ΔH_(RR)) to derive corresponding impulseresponses h_(LL), h_(RL), h_(LR) and h_(RR) (or their estimated errorsΔh_(LL), Δh_(RL), Δh_(LR) and Δh_(RR)), and outputs them from outputends 588, 590, 592 and 594, respectively. As shown in FIG. 32, thetransfer function calculating means 584 and the inverse FFT means 586may be exchanged therebetween in their positions.

[0384] With respect to the thus structured stereo echo canceller 16, 24of FIG. 28, the results of carrying out adaptive type operationsimulations are shown in FIGS. 33 to 48 in connection with one audiotransfer system. FIGS. 33 to 40 show time-domain variations in echocancellation amount, while FIGS. 41 to 48 show time-domain variations intransfer function estimated error. Herein, the simulations wereconducted under the following conditions.

[0385] Sampling Frequency: 11.025 kHz

[0386] The Number of Samples in One Frame: 4096 samples

[0387] The Number of Frames in One Block: variable (2 frames, 4 frames,8 frames, 16 frames)

[0388] Update Period of Filter Characteristic: per block (about 0.75seconds in case of the number of frames in one block being two, about1.5 seconds in case of 4 frames, about 3 seconds in case of 8 frames,about 6 seconds in case of 16 frames)

[0389] The Mean Number of Times of Ensemble Averaging in One Block: 31times in case of the number of frames in one block being two, 63 timesin case of 4 frames, 127 times in case of 8 frames, 255 times in case of16 frames {For the purpose of increasing the mean number of times, asshown in FIG. 49, one frame is divided into 16 intervals, and datacorresponding to one frame is extracted from the head of each divisionalinterval while overlapping data extraction successively, thereby toderive individual parameter values to be ensemble-averaged, and thederived individual parameter values are respectively ensemble-averagedin one block. Therefore, the mean number of times N of ensembleaveraging becomes such that N=(16×the number of frames in one block−1).}

[0390] In any case, the filter characteristics are not set in the firstblock, and the initial setting is implemented in the second block, andthereafter, updating is performed per block. The axis of ordinates (dB)is defined such that 0 dB represents the initial state where the filtercharacteristics are not set. Differences in simulation condition withrespect to FIGS. 33 to 48 are as follows. TABLE 2 Presence/AbsenceNumber of Frames Figure Number of Double Talk in One Block FIG. 33, FIG.41 Absent 2 FIG. 34, FIG. 42 4 FIG. 35, FIG. 43 8 FIG. 36, FIG. 44 16FIG. 37, FIG. 45 Present 2 FIG. 38, FIG. 46 4 FIG. 39, FIG. 47 8 FIG.40, FIG. 48 16

[0391] The simulation results of FIGS. 33 to 48 are considered.

[0392] (1) In Case of Absence of Double Talk

[0393] Since the rising speed of the echo cancellation amount is FIG.33>FIG. 34>FIG. 35>FIG. 36, the rising speed of the echo cancellationamount becomes faster as one block (update period) becomes shorter (thenumber of frames per block becomes smaller). When the number of framesin one block is 2, 4 or 8, the echo cancellation amount of about 25 dBis obtained (FIG. 33, 34 or 35). When the number of frames in one blockincreases, i.e. 16 frames, the echo cancellation amount requires a longtime for reduction thereof (FIG. 36). Since the estimated errorconvergence speed is FIG. 41>FIG. 42>FIG. 43>FIG. 44, the estimatederror convergence speed becomes faster as one block becomes shorter.

[0394] (2) In Case of Presence of Double Talk

[0395] When the number of frames in one block is 2 or 4 frames, the echocancellation amount is not increased (FIG. 37 or 38), and the estimatederror is not converged (FIG. 45 or 46) and thus can not be estimated.When the number of frames in one block is 8, the echo cancellationamount of about 15 dB is obtained (FIG. 39), and the estimated error isconverged to about −6 dB (FIG. 47). When the number of frames in oneblock is 16, the echo cancellation amount of about 17 dB is obtained(FIG. 40), and the estimated error is converged to about 10 dB and thusthe fairly stable estimation can be achieved.

[0396] From the foregoing simulation results, the followings can besaid.

[0397] (a) When the double talk is not detected, the convergence of theestimated error can be quickened by relatively shortening the updateperiod of the filter characteristic.

[0398] (b) When the double talk is detected, the estimated error can befully converged by relatively prolonging the update period of the filtercharacteristic.

[0399] Therefore, as described above, the transfer function calculatingmeans 502 of FIG. 28 makes relatively longer the update period of thefilter characteristics of the filter means 40-1 to 40-4 while the doubletalk is detected, whereas makes relatively shorter the update period ofthe filter characteristics while the double talk is not detected. Thismakes it possible to fully converge the estimated errors when the doubletalk exists, and further, quicken the convergence of the estimatederrors when there is no double talk.

[0400]FIG. 50 shows a modification of the stereo echo canceller 16, 24of FIG. 28. The same symbols are used with respect to those portionscommon to FIG. 28. In this modification, an orthogonalizing filter isdisposed on signal lines of loudspeakers SP(L) and SP(R). An inversefilter 596 has an inverse characteristic of the orthogonalizing filter500 {the inverse filter characteristic V of the foregoing equation(96)}, thereby to restore output signals x_(max) and x_(min) of theorthogonalizing filter 500 to the original signal x_(L) and x_(R), andfeeds them to the loudspeakers SP(L) and SP(R).

[0401] In the foregoing embodiments, the number of the loudspeakers istwo and the number of the microphones is two. However, it may also beconfigured that the number of the loudspeakers is two, while the numberof the microphones is one. FIG. 51 shows a structural example as aresult of modifying FIG. 1 in such a manner. The same symbols are usedwith respect to those portions common to FIG. 1. Left/right two-channelstereo signals x_(L) and x_(R) transmitted from the spot on thecounterpart side and inputted into line input ends LI(L) and LI(R) areoutputted from sound output ends SO(L) and SO(R) as they are (i.e. notthrough sum/difference signal producing means 52), and reproduced atloudspeakers SP(L) and SP(R), respectively.

[0402] Filter means 40-1 is set with an impulse response correspondingto a transfer function H_(L) between the loudspeaker SP(L) and amicrophone MC and performs, using such an impulse response, aconvolution calculation of a signal x_(L) to be outputted from the soundoutput end SO(L), thereby producing an echo cancel signal EC1corresponding to a signal y_(L) obtained such that the signal x_(L)outputted from the sound output end SO(L) is reproduced at theloudspeaker SP(L), collected by the microphone MC and inputted into asound input end SI. Filter means 40-3 is set with an impulse responsecorresponding to a transfer function H_(R) between the loudspeaker SP(R)and the microphone MC and performs, using such an impulse response, aconvolution calculation of a signal x_(R) to be outputted from the soundoutput end SO(R), thereby producing an echo cancel signal EC3corresponding to a signal y_(R) obtained such that the signal x_(R)outputted from the sound output end SO(R) is reproduced at theloudspeaker SP(R), collected by the microphone MC and inputted into thesound input end SI. An adder 44 performs a calculation of EC1+EC3. Asubtracter 48 subtracts an echo cancel signal EC1+EC3 from a collectedaudio signal y (=y_(L)+y_(R)) of the microphone MC inputted from thesound input end SI, thereby to perform echo cancellation. Anecho-canceled signal e (=e_(L)+e_(R)) is outputted from a line outputend LO and transmitted toward the spot on the counterpart side.

[0403] The sum/difference signal producing means 52 performs addition,using an adder 54, of the left/right two-channel stereo signals x_(L)and x_(R) inputted into the line input ends LI(L) and LI(R) so as toproduce a sum signal x_(M) (=x_(L)+x_(R)) while performs subtractionthereof using a subtracter 56 so as to produce a difference signal x_(S){=x_(L)−x_(R) (or it may also be x_(R)−x_(L))}. Transfer functioncalculating means 58 implements a cross-spectrum calculation between thesum signal x_(M) and the difference signal x_(S) produced by thesum/difference signal producing means 52 and the signal e outputted fromthe subtracter 48 and, based on this cross-spectrum calculation, setsfilter characteristics (impulse responses) of the filter means 40-1 and40-3. Specifically, upon starting the system, the filter characteristicsof the filter means 40-1 and 40-3 are not set, i.e. coefficients are allset to zero, so that the echo cancel signals EC1 and EC3 are zero, andthus the collected audio signal of the microphone MC itself is outputtedfrom the subtracter 48. Therefore, at this time, the transfer functioncalculating means 58 performs the cross-spectrum calculation between thesum signal x_(M) and the difference signal x_(S) produced by thesum/difference signal producing means 52 and the collected audio signale of the microphones MC outputted from the subtracter 48 and, based onthis cross-spectrum calculation, derives transfer functions of two audiotransfer systems between the loudspeakers SP(L) and SP(R) and themicrophone MC, respectively, and implements initial setting of thefilter characteristics of the filter means 40-1 and 40-3 to valuescorresponding to such transfer functions. After the initial setting,since the echo cancel signals are produced by the filter means 40-1 and40-3, the echo cancel error signal e corresponding to a differencesignal between the collected audio signal of the microphone MC and theecho cancel signal EC1+EC3 is outputted from the subtracter 48.Therefore, at this time, the transfer function calculating means 58performs the cross-spectrum calculation between the sum signal x_(M) andthe difference signal x_(S) produced by the sum/difference signalproducing means 52 and the echo cancel error signal e outputted from thesubtracter 48 and, based on this cross-spectrum calculation, derivesestimated errors of the transfer functions of the two audio transfersystems between the loudspeakers SP(L) and SP(R) and the microphone MC,respectively, and updates the filter characteristics of the filter means40-1 and 40-3 to values that cancel the estimated errors, respectively.By repeating this updating operation per prescribed time period, theecho cancel error can be converged to a minimum value. Further, even ifthe transfer functions change due to movement of the microphonepositions or the like, the echo cancel error can be converged to aminimum value by sequentially updating the filter characteristics of thefilter means 40-1 and 40-3 depending thereon.

[0404] Correlation detecting means 60 detects a correlation between thesum signal x_(M) and the difference signal x_(S) based on a correlationvalue calculation or the like, and stops updating of the foregoingfilter characteristics when the correlation value is no less than aprescribed value. When the correlation value becomes lower than theprescribed value, updating of the foregoing filter characteristics isrestarted. Also in the embodiments other than FIG. 1, it can beconfigured that the number of the loudspeakers is two, while the numberof the microphones is one.

[0405] The stereo echo canceller 16, 24 shown in each of the foregoingembodiments can be formed by the dedicated hardware or can also berealized through software processing in a general computer. For example,the functions of the respective blocks as shown in FIG. 1 and so forthcan be accomplished by a CPU (Central Processing Unit) and storing meanssuch as a RAM or ROM constituting the computer. Namely, the CPU may becaused to function as the echo canceller according to a program storedin the storing means such as the ROM or RAM.

[0406] In the foregoing embodiments, the description has been made aboutthe case where the two-channel stereo signals are handled. However, theecho cancellation can also be implemented using the technique of thisinvention with respect to those signals of three channels or more havinga correlation with each other.

What is claimed is:
 1. A multi-channel echo cancel method associated toa space provided therein with a plurality of loudspeakers and one or aplurality of microphones for forming a plurality of audio transfersystems, the method comprising: inputting multi-channel audio signalsfrom an outside, which have a correlation with each other, and which arereproduced by said respective loudspeakers and collected by saidmicrophones through the audio transfer systems; estimating individualtransfer functions of said plurality of said audio transfer systems or aplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions so as to set corresponding filtercharacteristics, respectively; producing echo cancel signalsrespectively by applying said set filter characteristics tocorresponding ones said the multi-channel audio signals to be reproducedby said respective loudspeakers or a plurality of composite signalsobtained by suitably combining said multi-channel audio signals; andsubtracting said echo cancel signals from corresponding individualcollected audio signals of said one or plurality of microphones, or froma plurality of composite signals obtained by suitably combining saidindividual collected audio signals, thereby performing echocancellation, wherein, reference signals are determined as a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that among saidmulti-channel audio signals, for deriving said individual transferfunctions of the respective audio transfer systems or said plurality ofsaid composite transfer functions obtained by suitably combining saidindividual transfer functions, thereby setting said corresponding filtercharacteristics.
 2. A multi-channel echo cancel method as recited inclaim 1, wherein calculation is conducted for respectively deriving theindividual transfer functions of the respective audio transfer systemsor the plurality of the composite transfer functions obtained bysuitably combining said individual transfer functions with using the setof the plurality of the low-correlation composite signals as thereference signals, such that the calculation is based on across-spectrum calculation between the plurality of the low-correlationcomposite signals and the individual collected audio signals of themicrophones, or the plurality of the composite signals obtained bysuitably combining said individual collected audio signals.
 3. Amulti-channel echo cancel method associated to a space provided thereinwith a plurality of loudspeakers and one or a plurality of microphonesfor forming a plurality of audio transfer systems through whichmulti-channel audio signals having a correlation with each other arereproduced by said respective loudspeakers and are collected by saidmicrophones, the method comprising: estimating individual transferfunctions of said plurality of said audio transfer systems or aplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions so as to set corresponding filtercharacteristics, respectively; producing echo cancel signalsrespectively by applying said set filter characteristics tocorresponding ones of said multi-channel audio signals to be reproducedby said respective loudspeakers or a plurality of composite signalsobtained by suitably combining said multi-channel audio signals; andsubtracting said echo cancel signals from corresponding individualcollected audio signals of said one or plurality of microphones, or froma plurality of composite signals obtained by suitably combining saidindividual collected audio signals, thereby performing echocancellation, wherein, reference signals are determined as a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that among saidmulti-channel audio signals, for deriving said individual transferfunctions of the respective audio transfer systems or said plurality ofsaid composite transfer functions obtained by suitably combining saidindividual transfer functions, thereby setting said corresponding filtercharacteristics, wherein calculation is conducted for respectivelyderiving the individual transfer functions of the respective audiotransfer systems or the plurality of the composite transfer functionsobtained by suitably combining said individual transfer functions withusing the set of the plurality of the low-correlation composite signalsas the reference signals, such that the calculation is based on across-spectrum calculation between the plurality of the low-correlationcomposite signals and the individual collected audio signals of themicrophones, or the plurality of the composite signals obtained bysuitably combining said individual collected audio signals, and whereinthe calculation of respectively deriving the individual transferfunctions of said plurality of the audio transfer systems or theplurality of the composite transfer functions obtained by suitablycombining said individual transfer functions is performed by combiningsaid multi-channel audio signals through addition or subtraction toproduce a plurality of low-correlation composite signals having a lowercorrelation with each other than that among said multi-channel audiosignals, deriving cross spectra by the cross-spectrum calculationbetween said plurality of the low-correlation composite signals and theindividual collected audio signals of the microphones, or the pluralityof the composite signals obtained by suitably combining said individualcollected audio signals, and ensemble-averaging each of the crossspectra in a predetermined time period for deriving the individualtransfer functions of said plurality of the audio transfer systems orthe plurality of the composite transfer functions obtained by suitablycombining said individual transfer functions.
 4. A multi-channel echocancel method associated to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones for forming aplurality of audio transfer systems, the method comprising: inputtingmulti-channel audio signals from an outside, which have a correlationwith each other, and which are reproduced by said respectiveloudspeakers and collected by said microphones through the audiotransfer systems; estimating individual transfer functions of saidplurality of said audio transfer systems or a plurality of compositetransfer functions obtained by suitably combining said individualtransfer functions so as to set corresponding filter characteristics,respectively; producing echo cancel signals respectively by applyingsaid set filter characteristics to corresponding ones of saidmulti-channel audio signals to be reproduced by said respectiveloudspeakers or a plurality of composite signals obtained by suitablycombining said multi-channel audio signals; and subtracting said echocancel signals from corresponding individual collected audio signals ofsaid one or plurality of microphones, or from a plurality of compositesignals obtained by suitably combining said individual collected audiosignals, thereby performing echo cancellation, wherein, referencesignals are determined as a set of a plurality of low-correlationcomposite signals which correspond to signals obtained by suitablycombining said multi-channel audio signals and which have a lowercorrelation with each other than that among said multi-channel audiosignals, for deriving estimated errors of said individual transferfunctions of the respective audio transfer systems or said plurality ofsaid composite transfer functions obtained by suitably combining saidindividual transfer functions, thereby updating said correspondingfilter characteristics to values that cancel the estimated errors.
 5. Amulti-channel echo cancel method as recited in claim 4, whereincalculation is conducted for respectively deriving the estimated errorsof the individual transfer functions of the respective audio transfersystems or the plurality of the composite transfer functions obtained bysuitably combining said individual transfer functions with using the setof the plurality of the low-correlation composite signals as thereference signals, such that the calculation is based on across-spectrum calculation between the plurality of the low-correlationcomposite signals and echo cancel error signals obtained by subtractingthe echo cancel signals from the corresponding individual collectedaudio signals of said one or plurality of the microphones, or from theplurality of the composite signals obtained by suitably combining saidindividual collected audio signals.
 6. A multi-channel echo cancelmethod associated to a space provided therein with a plurality ofloudspeakers and one or a plurality of microphones for forming aplurality of audio transfer systems through which multi-channel audiosignals having a correlation with each other are reproduced by saidrespective loudspeakers and are collected by said microphones, themethod comprising: estimating individual transfer functions of saidplurality of said audio transfer systems or a plurality of compositetransfer functions obtained by suitably combining said individualtransfer functions so as to set corresponding filter characteristics,respectively; producing echo cancel signals respectively by applyingsaid set filter characteristics to corresponding ones of saidmulti-channel audio signals to be reproduced by said respectiveloudspeakers or a plurality of composite signals obtained by suitablycombining said multi-channel audio signals; and subtracting said echocancel signals from corresponding individual collected audio signals ofsaid one or plurality of microphones, or from a plurality of compositesignals obtained by suitably combining said individual collected audiosignals, thereby performing echo cancellation, wherein, referencesignals are determined as a set of a plurality of low-correlationcomposite signals which correspond to signals obtained by suitablycombining said multi-channel audio signals and which have a lowercorrelation with each other than that among said multi-channel audiosignals, for deriving estimated errors of said individual transferfunctions of the respective audio transfer systems or said plurality ofsaid composite transfer functions obtained by suitably combining saidindividual transfer functions, thereby updating said correspondingfilter characteristics to values that cancel the estimated errors,wherein calculation is conducted for respectively deriving the estimatederrors of the individual transfer functions of the respective audiotransfer systems or the plurality of the composite transfer functionsobtained by suitably combining said individual transfer functions withusing the set of the plurality of the low-correlation composite signalsas the reference signals, such that the calculation is based on across-spectrum calculation between the plurality of the low-correlationcomposite signals and echo cancel error signals obtained by subtractingthe echo cancel signals from the corresponding individual collectedaudio signals of said one or plurality of the microphones, or from theplurality of the composite signals obtained by suitably combining saidindividual collected audio signals, and wherein the calculation ofrespectively deriving the estimated errors of the individual transferfunctions of said plurality of the audio transfer systems or theplurality of the composite transfer functions obtained by suitablycombining said individual transfer functions is performed by combiningsaid multi-channel audio signals through addition or subtraction toproduce a plurality of low-correlation composite signals having a lowercorrelation with each other than that among said multi-channel audiosignals, deriving cross spectra by the cross-spectrum calculationbetween said plurality of the low-correlation composite signals and theecho cancel error signals obtained by subtracting the echo cancelsignals from the corresponding individual collected audio signals ofsaid one or plurality of the microphones, or from the plurality of thecomposite signals obtained by suitably combining said individualcollected audio signals, and ensemble-averaging each of the crossspectra in a predetermined time period for deriving the estimated errorsof the individual transfer functions of said plurality of the audiotransfer systems or the plurality of the composite transfer functionsobtained by suitably combining said individual transfer functions.
 7. Amulti-channel echo cancel method associated to a space provided thereinwith a plurality of loudspeakers and one or a plurality of microphonesfor forming a plurality of audio transfer systems through whichmulti-channel audio signals having a correlation with each other arereproduced by said respective loudspeakers and are collected by saidmicrophones, the method comprising: estimating individual transferfunctions of said plurality of said audio transfer systems or aplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions so as to set corresponding filtercharacteristics, respectively; producing echo cancel signalsrespectively by applying said set filter characteristics tocorresponding ones of said multi-channel audio signals to be reproducedby said respective loudspeakers or a plurality of composite signalsobtained by suitably combining said multi-channel audio signals; andsubtracting said echo cancel signals from corresponding individualcollected audio signals of said one or plurality of microphones, or froma plurality of composite signals obtained by suitably combining saidindividual collected audio signals, thereby performing echocancellation, wherein, reference signals are determined as a set of aplurality of low-correlation composite signals which correspond tosignals obtained by suitably combining said multi-channel audio signalsand which have a lower correlation with each other than that among saidmulti-channel audio signals, for deriving estimated errors of saidindividual transfer functions of the respective audio transfer systemsor said plurality of said composite transfer functions obtained bysuitably combining said individual transfer functions, thereby updatingsaid corresponding filter characteristics to values that cancel theestimated errors, and wherein the correlation between said plurality ofsaid low-correlation composite signals is detected and, when a value ofsaid correlation is no less than a predetermined value, the updating ofsaid filter characteristics is suspended.
 8. A multi-channel soundtransfer method associated to two spaces each forming said plurality ofsaid audio transfer systems, wherein the multi-channel echo cancelmethod recited in claim 1 is carried out respectively in the two spaces,so that the multi-channel audio signals, which have been echo-canceledby performing said method, are transmitted between said two spaces.
 9. Amulti-channel echo cancel method associated to a space provided thereinwith a plurality of loudspeakers and one or a plurality of microphonesfor forming a plurality of audio transfer systems, the methodcomprising: inputting multi-channel audio signals from an outside, whichhave a correlation with each other, and which are reproduced by saidrespective loudspeakers and collected by said microphones through theaudio transfer systems; estimating transfer functions of said pluralityof said audio transfer systems so as to set corresponding filtercharacteristics, respectively; producing echo cancel signalsrespectively by applying said filter characteristics to correspondingmulti-channel audio signals to be reproduced by said respectiveloudspeakers; and subtracting said echo cancel signals fromcorresponding collected audio signals of said one or plurality of saidmicrophones, thereby performing echo cancellation, wherein a principalcomponent analysis is applied to said multi-channel audio signals toproduce a plurality of uncorrelated composite signals that areorthogonal to each other, and the transfer functions of the respectiveaudio transfer systems are respectively derived using a set of saidplurality of said uncorrelated composite signals as reference signals,thereby to set the corresponding filter characteristics.
 10. Amulti-channel echo cancel method associated to a space provided thereinwith a plurality of loudspeakers and one or a plurality of microphonesfor forming a plurality of audio transfer systems, the methodcomprising: inputting multi-channel audio signals from an outside, whichhave a correlation with each other, and which are reproduced by saidrespective loudspeakers and collected by said microphones through theaudio transfer systems; estimating transfer functions of said pluralityof said audio transfer systems so as to set corresponding filtercharacteristics, respectively; producing echo cancel signalsrespectively by applying said filter characteristics to correspondingmulti-channel audio signals to be reproduced by said respectiveloudspeakers; and subtracting said echo cancel signals fromcorresponding collected audio signals of said one or plurality of saidmicrophones, thereby performing echo cancellation, wherein a principalcomponent analysis is applied to said multi-channel audio signals toproduce a plurality of uncorrelated composite signals that areorthogonal to each other, and estimated errors of the transfer functionsof the respective audio transfer systems are respectively derived usinga set of said plurality of said uncorrelated composite signals asreference signals, thereby to update said corresponding filtercharacteristics to values that cancel said estimated errors.
 11. Astereo echo canceller associated to a space provided therein with twoloudspeakers and two microphones for forming four audio transfer systemsthrough which stereo sounds are reproduced by said respectiveloudspeakers and are collected by said respective microphones, thecanceller comprising: first and second filter sections that are providedcorresponding to the first and second microphones for subjecting anaudio signal supplied to the first loudspeaker to convolutioncalculations so as to produce first and second echo cancel signals,respectively; third and fourth filter sections that are providedcorresponding to the first and second microphones for subjecting anotheraudio signal supplied to the second loudspeaker to convolutioncalculations so as to produce third and fourth echo cancel signals,respectively; a first subtracting section that performs echocancellation by subtracting said first and third echo cancel signalsfrom a collected audio signal of the first microphone; and a secondsubtracting section that performs echo cancellation by subtracting saidsecond and fourth echo cancel signals from another collected audiosignal of the second microphone, wherein said stereo echo cancellerfurther comprises a transfer function calculating section thatrespectively derives filter characteristics corresponding to transferfunctions of said four audio transfer systems based on a cross-spectrumcalculation between a sum signal and difference signal of stereo audiosignals to be reproduced by said respective loudspeakers and thecollected audio signals of said respective microphones, thereby to setsaid derived filter characteristics to corresponding ones of said firstto fourth filter sections, respectively.
 12. A stereo echo canceller asrecited in claim 11, further comprising: an input section that inputssaid stereo audio signals; a sum/difference signal producing sectionthat produces said sum signal and said difference signal of the stereoaudio signals inputted from said input section; and a main signaltransmission system that transmits the stereo audio signals inputtedfrom said input section to said respective loudspeakers without passingthrough said sum/difference signal producing section, wherein saidtransfer function calculating section derives the filter characteristicscorresponding to the transfer functions of said four audio transfersystems based on the cross-spectrum calculation between the sum signaland difference signal produced by said sum/difference signal producingsection and the collected audio signals of said respective microphones,and sets the derived filter characteristics to corresponding ones ofsaid first to fourth filter sections, respectively.
 13. A stereo echocanceller associated to a space provided therein with two loudspeakersand two microphones for forming four audio transfer systems throughwhich stereo sounds are reproduced by said respective loudspeakers andare collected by said respective microphones, the canceller comprising:first and second filter sections that are provided corresponding to thefirst and second microphones for subjecting an audio signal supplied tothe first loudspeaker to convolution calculations so as to produce firstand second echo cancel signals, respectively; third and fourth filtersections that are provided corresponding to the first and secondmicrophones for subjecting another audio signal supplied to the secondloudspeaker to convolution calculations so as to produce third andfourth echo cancel signals, respectively; a first subtracting sectionperforms echo cancellation by subtracting said first and third echocancel signals from a collected audio signal of the first microphone;and a second subtracting section that performs echo cancellation bysubtracting said second and fourth echo cancel signals from anothercollected audio signal of the second microphone, wherein said stereoecho canceller further comprises a transfer function calculating sectionrespectively derives estimated errors of transfer functions of said fouraudio transfer systems based on a cross-spectrum calculation betweenrespective one of a sum signal and a difference signal of stereo audiosignals to be reproduced by said respective loudspeakers and respectiveone of echo cancel error signals obtained by subtracting thecorresponding echo cancel signals from the collected audio signals ofsaid two microphones, thereby to update filter characteristics of saidfirst to fourth filter sections to values that cancel said estimatederrors, respectively.
 14. A stereo echo canceller as recited in claim13, further comprising: an input section that inputs said stereo audiosignals; a sum/difference signal producing section that produces saidsum signal and said difference signal of the stereo audio signalsinputted from said input section; and a main signal transmission systemthat transmits the stereo audio signals inputted from said input sectionto said respective loudspeakers without passing through saidsum/difference signal producing section, wherein said transfer functioncalculating section derives the estimated errors of the transferfunctions of said four audio transfer systems based on thecross-spectrum calculation between the sum signal and difference signalproduced by said sum/difference signal producing section and therespective echo cancel error signals, and updates the filtercharacteristics of said first to fourth filter sections to the valuesthat cancel said estimated errors, respectively.
 15. A stereo echocanceller as recited in claim 13, further comprising a correlationdetecting section that detects a correlation between the sum signal andthe difference signal of said stereo audio signals, and that stops theupdating of said filter characteristics when a value of said correlationis no less than a predetermined value.
 16. A stereo echo cancellerassociated to a space provided therein with two loudspeakers and twomicrophones for forming four audio transfer systems, the cancellercomprising: an input section that inputs audio signals of stereo soundsfrom an outside, which are reproduced by said respective loudspeakersand collected by said respective microphones through the audio transfersystems; first and second filter sections that is provided correspondingto the first and second microphones for subjecting an audio signalsupplied to the first loudspeaker to convolution calculations so as toproduce first and second echo cancel signals, respectively; third andfourth filter sections that are provided corresponding to the first andsecond microphones for subjecting another audio signal supplied to thesecond loudspeaker to convolution calculations so as to produce thirdand fourth echo cancel signals, respectively; a first subtractingsection that performs echo cancellation by subtracting said first andthird echo cancel signals from a collected audio signal of the firstmicrophone; and a second subtracting section that performs echocancellation by subtracting said second and fourth echo cancel signalsfrom another collected audio signal of the second microphone, whereinsaid stereo echo canceller further comprises a transfer functioncalculating section respectively derives filter characteristicscorresponding to transfer functions of said four audio transfer systemsbased on a cross-spectrum calculation between said collected audiosignals of said respective microphones and mutually orthogonal twouncorrelated composite signals produced by applying a principalcomponent analysis to stereo audio signals to be reproduced by saidrespective loudspeakers, thereby to set said derived filtercharacteristics to corresponding ones of said first to fourth filtersections, respectively.
 17. A stereo echo canceller as recited in claim16, further comprising: an orthogonalizing section that applies saidprincipal component analysis to the stereo audio signals inputted fromsaid input section to produce said mutually orthogonal two uncorrelatedcomposite signals; and a main signal transmission system that transmitsthe stereo audio signals inputted from said input section to saidrespective loudspeakers without passing through said orthogonalizingsection, wherein said transfer function calculating section derives thefilter characteristics corresponding to the transfer functions of saidfour audio transfer systems based on the cross-spectrum calculationbetween the two uncorrelated composite signals produced by saidorthogonalizing section and the collected audio signals of saidrespective microphone, and sets the derived filter characteristics tocorresponding ones of said first to fourth filter sections,respectively.
 18. A stereo echo canceller associated to a space providedtherein with two loudspeakers and two microphones for forming four audiotransfer systems, the canceller comprising: an input section that inputsstereo audio signals from an outside, which are reproduced by saidrespective loudspeakers and collected by said respective microphonesthrough the audio transfer systems; first and second filter sectionsthat are provided corresponding to the first and second microphones forsubjecting an audio signal supplied to the first loudspeaker toconvolution calculations so as to produce first and second echo cancelsignals, respectively; third and fourth filter sections that areprovided corresponding to the first and second microphones forsubjecting another audio signal supplied to the second loudspeaker toconvolution calculations so as to produce third and fourth echo cancelsignals, respectively; a first subtracting section performs echocancellation by subtracting said first and third echo cancel signalsfrom a collected audio signal of the first microphone; and a secondsubtracting section performs echo cancellation by subtracting saidsecond and fourth echo cancel signals from another collected audiosignal of the second microphone, wherein said stereo echo cancellerfurther comprises a transfer function calculating section thatrespectively derives estimated errors of transfer functions of said fouraudio transfer systems based on a cross-spectrum calculation betweenmutually orthogonal two uncorrelated composite signals produced byapplying a principal component analysis to stereo audio signals to bereproduced by said respective loudspeakers and respective echo cancelerror signals obtained by subtracting the corresponding echo cancelsignals from the collected audio signals of said two microphones,thereby to update filter characteristics of said first to fourth filtersections to values that cancel said estimated errors, respectively. 19.A stereo echo canceller as recited in claim 18, further comprising: anorthogonalizing section applies a principal component analysis to thestereo audio signals inputted from said input section to produce saidmutually orthogonal two uncorrelated composite signals; and a mainsignal transmission system that transmits the stereo audio signalsinputted from said input section to said respective loudspeakers withoutpassing through said orthogonalizing section, wherein said transferfunction calculating section derives the estimated errors of thetransfer functions of said four audio transfer systems based on thecross-spectrum calculation between the two uncorrelated compositesignals produced by said orthogonalizing section and the respective echocancel error signals, and updates the filter characteristics of saidfirst to fourth filter sections to the values that cancel said estimatederrors, respectively.
 20. A stereo echo canceller as recited in claim18, further comprising a double talk detecting section that is providedfor detecting double talk in which a sound other than that reproduced bysaid loudspeakers is inputted into said microphones, wherein saidtransfer function calculating section makes relatively longer an updateperiod of said filter characteristics when said double talk is detected,while makes relatively shorter the update period of said filtercharacteristics when said double talk is not detected.
 21. A stereosound transfer apparatus associated to two spaces each forming said fouraudio transfer systems, wherein the stereo echo canceller recited inclaim 11 is arranged in each space, so that the stereo audio signals,which have been echo-canceled by said stereo echo cancellers, aretransmitted between said two spaces.
 22. A stereo echo cancellerassociated to a space provided therein with two loudspeakers and twomicrophones for forming four audio transfer systems through which stereosounds are reproduced by said respective loudspeakers and are collectedby said respective microphones, the canceller comprising: first andsecond filter sections that are provided for subjecting a sum signal ofstereo audio signals to be reproduced by said respective loudspeakers toconvolution calculations respectively, so as to produce first and secondecho cancel signals; third and fourth filter sections that are providedfor subjecting a difference signal of the stereo audio signals to bereproduced by said respective loudspeakers to convolution calculationsrespectively, so as to produce third and fourth echo cancel signals; afirst subtracting section that performs echo cancellation by subtractingsaid first and third echo cancel signals from a collected audio signalof the first microphone; and a second subtracting section that performsecho cancellation subtracting said second and fourth echo cancel signalsfrom another collected audio signal of the second microphone, whereinsaid stereo echo canceller further comprises a transfer functioncalculating section respectively derives filter characteristicscorresponding to composite transfer functions of said four audiotransfer systems based on a cross-spectrum calculation between the sumsignal and difference signal of the stereo audio signals to bereproduced by said respective loudspeakers and the collected audiosignals of said respective microphones, thereby to set said derivedfilter characteristics to corresponding ones of said first to fourthfilter sections, respectively.
 23. A stereo echo canceller as recited inclaim 22, further comprising: an input section that inputs said stereoaudio signals; a sum/difference signal producing section that producessaid sum signal and difference signal of the stereo audio signalsinputted from said input section; and a main signal transmission systemthat transmits the stereo audio signals inputted from said input sectionto said respective loudspeakers without passing through saidsum/difference signal producing section, wherein said transfer functioncalculating section derives the filter characteristics corresponding tothe composite transfer functions of said four audio transfer systemsbased on the cross-spectrum calculation between the sum signal anddifference signal produced by said sum/difference signal producingsection and the collected audio signals of said respective microphones,and sets the derived filter characteristics to corresponding ones ofsaid first to fourth filter sections, respectively.
 24. A stereo echocanceller associated to a space provided therein with two loudspeakersand two microphones for forming four audio transfer systems throughwhich stereo sounds are reproduced by said respective loudspeakers andare collected by said respective microphones, the canceller comprising:first and second filter sections that subjects a sum signal of stereoaudio signals to be reproduced by said respective loudspeakers toconvolution calculations respectively, so as to produce first and secondecho cancel signals; third and fourth filter sections that subjects adifference signal of the stereo audio signals to be reproduced by saidrespective loudspeakers to convolution calculations respectively, so asto produce third and fourth echo cancel signals; a first subtractingsection that performs echo cancellation by subtracting said first andthird echo cancel signals from a collected audio signal of the firstmicrophone; and a second subtracting section that performs echocancellation by subtracting, said second and fourth echo cancel signalsfrom another collected audio signal of the second microphone, whereinsaid stereo echo canceller further comprises a transfer functioncalculating section that respectively derives estimated errors ofcomposite transfer functions of said four audio transfer systems basedon a cross-spectrum calculation between the sum signal and differencesignal of the stereo audio signals to be reproduced by said respectiveloudspeakers and respective echo cancel error signals obtained bysubtracting the corresponding echo cancel signals from the collectedaudio signals of said two microphones, thereby to update filtercharacteristics of said first to fourth filter sections to values thatcancel said estimated errors, respectively.
 25. A stereo echo cancelleras recited in claim 24, further comprising: an input section that inputssaid stereo audio signals; a sum/difference signal producing sectionthat produces said sum signal and difference signal of the stereo audiosignals inputted from said input section; and a main signal transmissionsystem that transmits the stereo audio signals inputted from said inputsection to said respective loudspeakers without passing through saidsum/difference signal producing section, wherein said transfer functioncalculating section derives the estimated errors of the compositetransfer functions of said four audio transfer systems based on thecross-spectrum calculation between the sum signal and difference signalproduced by said sum/difference signal producing section and therespective echo cancel error signals, and updates the filtercharacteristics of said first to fourth filter sections to the valuesthat cancel said estimated errors, respectively.
 26. A stereo echocanceller as recited in claim 24, further comprising a correlationdetecting section that is provided for detecting a correlation betweenthe sum signal and the difference signal of said stereo audio signals,and for stopping the updating of said filter characteristics when avalue of said correlation is no less than a predetermined value.
 27. Astereo sound transfer apparatus associated to two spaces each formingsaid four audio transfer systems, wherein the stereo echo cancellerrecited in claim 22 is arranged in each space, so that the stereo audiosignals, which have been echo-canceled by said stereo echo cancellers,are transmitted between said two spaces.
 28. A stereo echo cancellerassociated to a space provided therein with two loudspeakers and twomicrophones for forming four audio transfer systems through which stereosounds are reproduced by said respective loudspeakers and are collectedby said respective microphones, the canceller comprising: first andsecond filter sections that subject an audio signal supplied to thefirst loudspeaker to convolution calculations respectively, so as toproduce first and second echo cancel signals; third and fourth filtersections that subject another audio signal supplied to the secondloudspeaker to convolution calculations respectively, so as to producethird and fourth echo cancel signals; a first subtracting section thatperforms echo cancellation by subtracting said first and third echocancel signals from a sum signal of collected audio signals of therespective microphones; and a second subtracting section that performsecho cancellation subtracting said second and fourth echo cancel signalsfrom a difference signal of the collected audio signals of therespective microphones, wherein said stereo echo canceller furthercomprises a transfer function calculating section that respectivelyderives filter characteristics corresponding to composite transferfunctions of said four audio transfer systems based on a cross-spectrumcalculation between a sum signal and difference signal of stereo audiosignals to be reproduced by said respective loudspeakers and the sumsignal and difference signal of the collected audio signals of saidrespective microphones, thereby to set said derived filtercharacteristics to corresponding ones of said first to fourth filtersections, respectively.
 29. A stereo echo canceller associated to aspace provided therein with two loudspeakers and two microphones forforming four audio transfer systems through which stereo sounds arereproduced by said respective loudspeakers and are collected by saidrespective microphones, the canceller comprising: first and secondfilter sections that subject an audio signal supplied to the firstloudspeaker to convolution calculations so as to produce first andsecond echo cancel signals; third and fourth filter sections thatsubject another audio signal supplied to the second loudspeaker toconvolution calculations respectively, so as to produce third and fourthecho cancel signals; first subtracting section that performs echocancellation by subtracting said first and third echo cancel signalsfrom a sum signal of collected audio signals of the respectivemicrophones; and second subtracting section that performs echocancellation by subtracting said second and fourth echo cancel signalsfrom a difference signal of the collected audio signals of therespective microphones, wherein said stereo echo canceller furthercomprises a transfer function calculating section that respectivelyderives estimated errors of composite transfer functions of said fouraudio transfer systems based on a cross-spectrum calculation between asum signal and difference signal of stereo audio signals to bereproduced by said respective loudspeakers and respective echo cancelerror signals obtained by subtracting the corresponding echo cancelsignals from the sum signal and difference signal of the collected audiosignals of said respective microphones, thereby to update filtercharacteristics of said first to fourth filter sections to values thatcancel said estimated errors, respectively.
 30. A stereo echo cancelleras recited in claim 29, further comprising a correlation detectingsection that is provided for detecting a correlation between the sumsignal and the difference signal of said stereo audio signals, and forstopping the updating of said filter characteristics when a value ofsaid correlation is no less than a predetermined value.
 31. A stereosound transfer apparatus associated to two spaces each forming said fouraudio transfer systems, wherein the stereo echo canceller recited inclaim 28 is arranged in each space, so that the stereo audio signals,which have been echo-canceled by said stereo echo cancellers, aretransmitted between said two spaces.
 32. A stereo echo cancellerassociated to a space provided therein with two loudspeakers and twomicrophones for forming four audio transfer systems through which stereosounds are reproduced by said respective loudspeakers and are collectedby said respective microphones, the canceller comprising: first andsecond filter sections that subject a sum signal of stereo audio signalsto be reproduced by said respective loudspeakers to convolutioncalculations respectively, so as to produce first and second echo cancelsignals; third and fourth filter sections that subject a differencesignal of the stereo audio signals to be reproduced by said respectiveloudspeakers to convolution calculations respectively, so as to producethird and fourth echo cancel signals; a first subtracting section thatperforms echo cancellation by subtracting said first and third echocancel signals from a sum signal of collected audio signals of therespective microphones; and a second subtracting section that performsecho cancellation by subtracting said second and fourth echo cancelsignals from a difference signal of the collected audio signals of therespective microphones, wherein said stereo echo canceller furthercomprises a transfer function calculating section that respectivelyderives filter characteristics corresponding to composite transferfunctions of said four audio transfer systems based on a cross-spectrumcalculation between the sum signal and difference signal of the stereoaudio signals to be reproduced by said respective loudspeakers and thesum signal and difference signal of the collected audio signals of therespective microphones, thereby to set said derived filtercharacteristics to corresponding ones of said first to fourth filtersections, respectively.
 33. A stereo echo canceller associated to aspace provided therein with two loudspeakers and two microphones forforming four audio transfer systems through which stereo sounds arereproduced by said respective loudspeakers and are collected by saidrespective microphones, the canceller comprising: first and secondfilter sections that subject a sum signal of stereo audio signals to bereproduced by said respective loudspeakers to convolution calculationsrespectively, so as to produce first and second echo cancel signals;third and fourth filter sections that subject a difference signal of thestereo audio signals to be reproduced by said respective loudspeakers toconvolution calculations respectively, so as to produce third and fourthecho cancel signals; a first subtracting section that performs echocancellation by subtracting said first and third echo cancel signalsfrom a sum signal of collected audio signals of the respectivemicrophones; and a second subtracting section that performs echocancellation by subtracting said second and fourth echo cancel signalsfrom a difference signal of the collected audio signals of therespective microphones, wherein said stereo echo canceller furthercomprises a transfer function calculating section that respectivelyderives estimated errors of composite transfer functions of said fouraudio transfer systems based on a cross-spectrum calculation between thesum signal and difference signal of the stereo audio signals to bereproduced by said respective loudspeakers and respective echo cancelerror signals obtained by subtracting the corresponding echo cancelsignals from the sum signal and the difference signal of the collectedaudio signals of the respective microphones, thereby to update filtercharacteristics of said first to fourth filter sections to values thatcancel said estimated errors, respectively.
 34. A stereo echo cancelleras recited in claim 33, further comprising a correlation detectingsection that is provided for detecting a correlation between the sumsignal and the difference signal of said stereo audio signals, and forstopping the updating of said filter characteristics when a value ofsaid correlation is no less than a predetermined value.
 35. A stereosound transfer apparatus associated to two spaces each forming said fouraudio transfer systems, wherein the stereo echo canceller recited inclaim 32 is arranged in each space, so that the stereo audio signals,which have been echo-canceled by said stereo echo cancellers, aretransmitted between said two spaces.
 36. A transfer function calculationapparatus being associated to a space provided therein with a pluralityof loudspeakers and one or a plurality of microphones for forming aplurality of audio transfer systems, and being capable of estimatingindividual transfer functions of said plurality of audio transfersystems or a plurality of composite transfer functions obtained bysuitably combining said individual transfer functions, the apparatuscomprising: an input section that inputs multi-channel audio signalsfrom an outside, which have a correlation with each other, and which arereproduced by said respective loudspeakers and collected by saidmicrophones through the audio transfer systems; a providing section thatprovides reference signals as a set of a plurality of low-correlationcomposite signals which correspond to signals obtained by suitablycombining said multi-channel audio signals and which have a lowercorrelation with each other than that between said multi-channel audiosignals; and a calculation section that estimates the individualtransfer functions of the respective audio transfer systems or theplurality of the composite transfer functions obtained by suitablycombining said individual transfer functions based on the determinedreference signals.
 37. A transfer function calculation apparatus asrecited in claim 36, wherein the calculation section respectivelyderives the individual transfer functions of the respective audiotransfer systems or the plurality of said composite transfer functionsobtained by suitably combining said individual transfer functions, usingas the reference signals the set of the plurality of the low-correlationcomposite signals, such that calculation of respectively deriving theindividual transfer functions of the respective audio transfer systemsor the plurality of said composite transfer functions obtained bysuitably combining said individual transfer functions is based on across-spectrum calculation between the plurality of said low-correlationcomposite signals and individual collected audio signals of themicrophones, or a plurality of composite signals obtained by suitablycombining said individual collected audio signals.
 38. A transferfunction calculation apparatus being associated to a space providedtherein with a plurality of loudspeakers and one or a plurality ofmicrophones for forming a plurality of audio transfer systems throughwhich multi-channel audio signals having a correlation with each otherare reproduced by said respective loudspeakers and are collected by saidmicrophones, and being capable of estimating individual transferfunctions of said plurality of audio transfer systems or a plurality ofcomposite transfer functions obtained by suitably combining saidindividual transfer functions, the apparatus comprising: a providingsection that provides reference signals as a set of a plurality oflow-correlation composite signals which correspond to signals obtainedby suitably combining said multi-channel audio signals and which have alower correlation with each other than that between said multi-channelaudio signals; and a calculation section that estimates the individualtransfer functions of the respective audio transfer systems or theplurality of the composite transfer functions obtained by suitablycombining said individual transfer functions based on the determinedreference signals, wherein the calculation section respectively derivesthe individual transfer functions of the respective audio transfersystems or the plurality of said composite transfer functions obtainedby suitably combining said individual transfer functions, using as thereference signals the set of the plurality of the low-correlationcomposite signals, such that calculation of respectively deriving theindividual transfer functions of the respective audio transfer systemsor the plurality of said composite transfer functions obtained bysuitably combining said individual transfer functions is based on across-spectrum calculation between the plurality of said low-correlationcomposite signals and individual collected audio signals of themicrophones, or a plurality of composite signals obtained by suitablycombining said individual collected audio signals, and wherein theproviding section combines said multi-channel audio signals throughaddition or subtraction to produce said plurality of saidlow-correlation composite signals having a lower correlation with eachother than that between said multi-channel audio signals, such that thecalculation section derives cross spectra based on said cross-spectrumcalculation between said plurality of said low-correlation compositesignals and the individual collected audio signals of the microphones,or the plurality of composite signals obtained by suitably combiningsaid individual collected audio signals, and ensemble-averages therespective cross spectra in a predetermined time period for deriving theindividual transfer functions of said plurality of said audio transfersystems or the plurality of said composite transfer functions obtainedby suitably combining said individual transfer functions.
 39. A transferfunction calculation apparatus as recited in claim 37, wherein theproviding section produces a plurality of uncorrelated composite signalsmutually orthogonal as the reference signals by applying a principalcomponent analysis to said multi-channel audio signals, such that thecalculation section derives cross spectra based on said cross-spectrumcalculation between said plurality of said uncorrelated compositesignals and the individual collected audio signals of the microphones,and ensemble-averages the respective cross spectra in a predeterminedtime period for deriving the individual transfer functions of saidplurality of said audio transfer systems.
 40. A transfer functioncalculation apparatus being associated to a space provided therein withtwo loudspeakers and two microphones for forming four audio transfersystems, and being capable of estimating individual transfer functionsof said four audio transfer systems, the apparatus comprising: a sectionthat inputs stereo audio signals from an outside, which are reproducedby said respective loudspeakers and collected by said respectivemicrophones; a section that produces mutually orthogonal twouncorrelated composite signals by applying a principal componentanalysis to said stereo audio signals; and a section that estimates saidindividual transfer functions of said four audio transfer systems usinga set of said two uncorrelated composite signals as reference signals.41. An echo cancel method associated to a space provided therein with aplurality of loudspeakers and one or a plurality of microphones forforming a plurality of audio transfer systems through which audiosignals of multi-channels having a correlation with each other arereproduced by said respective loudspeakers and are collected by saidmicrophones, and designed for performing an echo cancellation bysubtracting an echo cancel signal from the audio signals collected bythe respective microphone or from composite signals obtained bycombining the collected audio signals, the method comprising: inputtinga plurality of low-correlation audio signals which are obtained bysuitably combining first audio signals of multi-channels and which havea lower correlation with each other than that among said first audiosignals of multi-channels; generating second audio signals ofmulti-channels having a correlation with each other by computation basedon the inputted low-correlation audio signals; feeding the generatedsecond audio signals to the respective loudspeakers so as to reproduceaudio sounds; feeding the generated second audio signals or the inputtedlow-correlation audio signals to filters; estimating individual transferfunctions of said plurality of said audio transfer systems or aplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions based on the inputted low-correlationaudio signals so as to set corresponding filter characteristics;producing echo cancel signals by applying said set filtercharacteristics to the second audio signals or the low-correlation audiosignals fed to the filters; and subtracting said echo cancel signalsfrom collected audio signals obtained by collecting the reproduced audiosounds by the microphones or from composite audio signals obtained bysuitably combining said collected audio signals, thereby performing theecho cancellation.
 42. An echo cancel method as recited in claim 41,wherein the inputted low-correlation audio signals are obtained byadding or subtracting the first audio signals of multi-channels witheach other.
 43. An echo cancel method associated to a space providedtherein with a plurality of loudspeakers and one or a plurality ofmicrophones for forming a plurality of audio transfer systems throughwhich audio signals of multi-channels having a correlation with eachother are reproduced by said respective loudspeakers and are collectedby said microphones, and designed for performing an echo cancellation bysubtracting an echo cancel signal from the audio signals collected bythe respective microphone or from composite signals obtained bycombining the collected audio signals, the method comprising: inputtinga plurality of first low-correlation audio signals which are obtained bysuitably combining first audio signals of multi-channels and which havea lower correlation with each other than that among said first audiosignals of multi-channels; generating second audio signals ofmulti-channels having a correlation with each other by computation basedon the inputted first low-correlation audio signals; feeding thegenerated second audio signals to the respective loudspeakers so as toreproduce audio sounds; generating second low-correlation audio signalsof multi-channels based on the generated second audio signals; feedingthe generated second audio signals or the generated secondlow-correlation audio signals to filters; estimating individual transferfunctions of said plurality of said audio transfer systems or aplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions based on the generated secondlow-correlation audio signals so as to set corresponding filtercharacteristics; producing echo cancel signals by applying said setfilter characteristics to the second audio signals or the secondlow-correlation audio signals fed to the filters; and subtracting saidecho cancel signals from collected audio signals obtained by collectingthe reproduced audio sounds at the microphones or from composite audiosignals obtained by suitably combining said collected audio signals,thereby performing the echo cancellation.
 44. An echo cancel method asrecited in claim 43, wherein the inputted first low-correlation audiosignals are obtained by adding or subtracting the first audio signals ofmulti-channels with each other.
 45. An echo canceller associated to aspace provided therein with a plurality of loudspeakers and one or aplurality of microphones for forming a plurality of audio transfersystems through which audio signals of multi-channels having acorrelation with each other are reproduced by said respectiveloudspeakers and are collected by said microphones, and designed forperforming an echo cancellation by subtracting an echo cancel signalfrom the audio signals collected by the respective microphone or fromcomposite signals obtained by combining the collected audio signals, theecho canceller comprising: an inputting section that inputs a pluralityof low-correlation audio signals which are obtained by suitablycombining first audio signals of multi-channels and which have a lowercorrelation with each other than that among said first audio signals ofmulti-channels; a demodulating section that is provided for generatingsecond audio signals of multi-channels having a correlation with eachother by demodulating the inputted low-correlation audio signals, andfor feeding the generated second audio signals to the respectiveloudspeakers so as to reproduce audio sounds; an estimating section thatestimates individual transfer functions of said plurality of said audiotransfer systems or a plurality of composite transfer functions obtainedby suitably combining said individual transfer functions based on theinputted low-correlation audio signals so as to set corresponding filtercharacteristics; a filter section that produces echo cancel signals byapplying said set filter characteristics to the second audio signals orthe low-correlation audio signals fed to the filter section; and asubtracting section that subtracts said echo cancel signals fromcollected audio signals obtained by collecting the reproduced audiosounds at the microphones or from composite audio signals obtained bysuitably combining said collected audio signals, thereby performing theecho cancellation.
 46. An echo canceller as recited in claim 45, whereinthe inputted low-correlation audio signals are obtained by adding orsubtracting the first audio signals of multi-channels with each other.47. An echo canceller associated to a space provided therein with aplurality of loudspeakers and one or a plurality of microphones forforming a plurality of audio transfer systems through which audiosignals of multi-channels having a correlation with each other arereproduced by said respective loudspeakers and are collected by saidmicrophones, and designed for performing an echo cancellation bysubtracting an echo cancel signal from the audio signals collected bythe respective microphone or from composite signals obtained bycombining the collected audio signals, the echo canceller comprising: aninputting section that inputs a plurality of first low-correlation audiosignals which are obtained by suitably combining first audio signals ofmulti-channels and which have a lower correlation with each other thanthat among said first audio signals of multi-channels; a demodulatingsection that is provided for generating second audio signals ofmulti-channels having a correlation with each other by demodulating theinputted first low-correlation audio signals, and for feeding thegenerated second audio signals to the respective loudspeakers so as toreproduce audio sounds; an estimating section that is provided forgenerating second low-correlation audio signals of multi-channels basedon the generated second audio signals, and for estimating individualtransfer functions of said plurality of said audio transfer systems or aplurality of composite transfer functions obtained by suitably combiningsaid individual transfer functions based on the generated secondlow-correlation audio signals so as to set corresponding filtercharacteristics; a filter section that produces echo cancel signals byapplying said set filter characteristics to the second audio signals orthe second low-correlation audio signals fed to the filter section; anda subtracting section that subtracts said echo cancel signals fromcollected audio signals obtained by collecting the reproduced audiosounds at the microphones or from composite audio signals obtained bysuitably combining said collected audio signals, thereby performing theecho cancellation.
 48. An echo canceller as recited in claim 47, whereinthe inputted first low-correlation audio signals are obtained by addingor subtracting the first audio signals of multi-channels with eachother.
 49. A multi-channel echo cancel method as recited in claim 1,wherein the multi-channel audio signals being inputted from an outsideand having a correlation with each other are reproduced by saidrespective loudspeakers without lowering the correlation of the inputtedmulti-channel audio signals.
 50. A multi-channel echo cancel method asrecited in claim 1, wherein the multi-channel audio signals beinginputted from an outside and having a correlation with each other areprovisionally modulated to lower the correlation, then demodulated torestore the correlation, and thereafter reproduced by said respectiveloudspeakers.
 51. A multi-channel echo cancel method as recited in claim50, wherein the multi-channel audio signals are provisionally modulatedto lower the correlation by adding and subtracting the multi-channelaudio signals with each other, or by orthogonalizing the multi-channelaudio signals with each other.
 52. An echo canceller being associated toa space provided therein with a plurality of loudspeakers and one or aplurality of microphones for forming a plurality of audio transfersystems, the echo canceller comprising: an input section that inputsmulti-channel audio signals from an outside, which have a correlationwith each other, and which are reproduced by said respectiveloudspeakers as audio sounds having correlation with each other; agenerating section that generates reference signals as a set oflow-correlation composite signals based on said multi-channel audiosignals, the low-correlation composite signals having a lowercorrelation with each other than that among said multi-channel audiosignals; an estimating section that estimates individual transferfunctions of the respective audio transfer systems or composite transferfunctions obtained by suitably combining said individual transferfunctions based on the generated reference signals so as to set filtercharacteristics; a filter section that produces echo cancel signals byapplying said set filter characteristics to the multi-channel audiosignals inputted from the outside or composite audio signals generatedbased on the inputted multi-channel audio signals; and a subtractingsection that subtracts said echo cancel signals from collected audiosignals obtained by collecting the reproduced audio sounds havingcorrelation with each other at the microphones or from composite audiosignals obtained based on said collected audio signals, therebyperforming the echo cancellation.